This noise can be heard as airflow hissing in the window area. It occurs mostly at high speeds, and changes in the vehicle speed or in the direction of the wind alter this noise. Usually the volume of the noise increases with vehicle speed, but the noise occasionally becomes inaudible depending on the wind direction.
Main causes
Air disturbances created by irregularities in the vehicle body
Air leaking through gaps between body panels

Mechanism
1.Noises created by air disturbances due to body irregularities
When air flows over protrusions or recesses on the vehicle surface, air disturbances are created behind them. The noises created by these disturbances then enter the passenger compartment through the doors or windows.
2.Noises created by leaking air
At high speeds, the air pressure outside the passenger compartment is lower than that of the inside. When this air inside is forced out through gaps in the weatherstrips or body panels, and comes into contact with the exterior air flow, noise is generated.
1.Wind noise may occur anywhere, but the four areas shown in the figure are known to be the more common sources.
2.Some of the countermeasures for wind noise are to apply body sealant to the body panel gaps, or to align the doors or windshields.
3.Modern vehicles have less body panel gaps, and thus less wind noise, than older model vehicles.

A relatively high-pitched noise is whining in the passenger compartment, increasing in volume with increases in engine rpm, when the vehicle is running at high speeds or under a heavy load.
1.Main causes
Mechanical noise from the engine
Combustion noise
Fan noise
Exhaust noise
Vibrations transmitted through air, engine mounts, hoses, and cables
2.Mechanics of noise development
The following illustration shows the route through which the noises and vibrations enter the passenger compartment.

1.Torque converter slippage
(1)Noise caused by imbalance
If there is an imbalance in the pump impeller, and another behind the turbine runner, such as in the propeller shaft, each one of these components will produce a vibration.
Because the torque converter by nature has slippage, the relative imbalance between the pump impeller and the propeller shaft will appear to rotate gradually around their common axis.
When the imbalance of the pump impeller is on the opposite side in relation to the imbalance of the propeller shaft, vibrations will be smaller than when the imbalances are both on the same side.
These two varying vibrations are transmitted to the body panels through the engine rear support members or suspension arms bushings to produce the beating noise.
(2)Noise caused by torque fluctuations
2.Vibrations due to transmission gear ratio
Besides the body beating noise created by torque converter slippage, the transmission gear ratio – such as overdrive in an automatic transmission, or 5th gear in manual -sometimes causes the engine vibration frequency to approach the drive train vibration frequency, resulting in body beating noise.
3.Vibrations due to pulley ratio
If the diameter of an accessory pulley and that of the crankshaft are close to integer multiples (e.g., 2 or 4) of each other, the differences in the vibration frequency of the engine and that of the accessory become small, and combine to produce body beating noises. When this is the case, pinpoint the source by removing the drive belts one at a time. This will enable you to isolate the problem pulley.
4.Vibrations due to differential reduction ratio

Mechanism
1.To determine whether a beating noise is caused by torque converter slippage or not, you only need to vary the pressure of your foot on the accelerator pedal. If this alters the frequency of the noise, you have isolated the source of the symptom.
2.A four-cylinder engine is more prone to body beating noise created by torque converter slippage when the joint angle of the propeller shaft is great.
3.Since body beating noises are created by the combination of two different vibrations, they can be eliminated by either increasing the difference between these two frequencies, or by reducing the vibration of either one.
4.When test driving for these body beating noises, do not accelerate or decelerate abruptly; if you do this, you will exceed the vehicle or engine speed at which the symptom may be duplicated.

The volume (amplitude) of this noise alters cyclically, coming to you in waves. The frequency of the noise accelerates with the increase in vehicle speed. This noise occurs at certain engine and/or vehicle speeds, within a relatively narrow range. Even weak body beating noises can be noticed easily because of their cyclic loud-and- soft pattern.
How a beating sound is created
There must be two separate sounds for the beating noise to occur:
1.When two separate sounds with slightly different frequencies, A and B, occur simultaneously, their volume varies cyclically, resulting in a beating noise.
2.When the hills (or the valleys) of two different vibration frequencies, A and B, overlap, they produce a louder noise. And when the hills and the valleys of two different vibration frequencies overlap, they produce a softer noise.
The beating noise frequency can be represented as follows:
Frequency A – Frequency B = Beating noise frequency
Those that we are most apt to sense are beating noises with frequencies of 2 to 6 cycles per second, or 2 to 6 Hz.
Although we have discussed body beating noise here in terms of compound sounds, the same concept of cyclic development may also be applied to vibrations, which are the source of sounds.
Main causes
Difference in engine and drive train vibration frequencies (torque converter slippage, or transmission gear ratio).
Difference in engine and tire vibrations (differential reduction ratio).
Differences in engine and auxiliary component vibration frequencies (pulley ratio of crankshaft pulley to accessory pulleys).

This type of noise is felt as a pressure in your ears, and its origin is often unknown to you. It increases in pitch (frequency) with the vehicle speed. It occurs within a relatively narrow vehicle speed range centered around 10 km/h, or around 50 rpm if associated with engine speed.
Main causes
Out-of-tune engine
Inertia due to reciprocating motion of pistons, or imbalance in engine
Unbalanced propeller shaft
Incorrect propeller shaft joint angle
Resonances in the exhaust pipe
Exhaust noise
Resonances in auxiliary engine components
Vibration due to torsional stresses on the propeller shaft and drive shafts.

Mechanism
1.Propeller shaft joint angle
When a joint angle exists in the propeller shaft, there are two torque fluctuations for every revolution of the propeller shaft. These fluctuations become larger as the joint angle increases.
At certain vehicle speeds these torque fluctuations vibrate the drive train, and are transmitted through the rear suspension arm bushings or springs, causing the body panels to vibrate. This results in the body booming noise.
2.Out-of-balance propeller shaft
When the propeller shaft is out of balance, the centrifugal force created by this imbalance attempts to cause the ends of the propeller shaft to bend outward and revolve in large circles around the centerline of the shaft. Of course this does not actually happen because the propeller shaft is fixed at both ends. However, it does cause the shaft to vibrate once each time it rotates.
This vibratory force attempts to bend the propeller shaft.
This vibratory force is further transmitted through the engine rear mounts, center bearing for the propeller shaft, rear suspension bushings, and to the body panels. The body panels then vibrate, generating the body booming noise.
3.Exhaust pipe vibrations
The exhaust pipe, being long and narrow, is easily vibrated. Another important factor which causes it to vibrate vigorously is that it is attached to the engine, the largest source of vibrations in a vehicle.
When the exhaust pipe resonates with engine vibration, the vibration is further amplified and transmitted by way of the O-rings and muffler clamps to the vehicle body, causing body booming noise. To isolate the problem area you could remove the O-rings one by one.
4.Vibration of auxiliary components
Body booming noise may be related to either the engine speed or to a certain vehicle speed.
This can be determined by the following methods:
1.Does it occur when the engine is revved?
2.Does it occur when coasting?

This type of noise occurs often in vehicle with block-or lug-pattern tires when driving on paved roads. It increases in volume with the vehicle speed.
1.Main cause
Air-pumping effects of the tread pattern.
2.Mechanics of noise development
(1)When a tire turns on the road surface, air gets trapped between the tread grooves, and the air is compressed upon contact with the road surface.
(2)As the tire keeps spinning, the compressed air is then freed from the tread – this is called “air-pumping action” – and expands when released, creating a popping sound. The multiplication of these sounds produces a roaring noise.
Tires with patterns which easily trap air have a tendency to produce louder roars. In other words, the block or lug tread pattern is more likely to generate noise than rib pattern.

A continuous rumble or roar at a constant pitch makes up road noise; the volume of this noise increases with the vehicle speed. This occurs mainly when driving on poorly-paved roads.
1.Main causes
Small bumps or depressions in the road surface
Tires resonating with these irregularities
Vibration of the suspension (due to improper spring constants of the rubber bushings)
2.Mechanics of noise development
(1)When driving over poorly-paved roads, small vibrations are generated in the tires. When these vibrations reach a certain frequency, they produce resonances in the tires which amplify the vibrations.
(2)These vibrations are transmitted from the suspension to the vehicle body, and a roaring is produced from the body panels.

By harshness, we mean a single-stroke impact such as one produced by striking an object with a hammer. Along with a loud noise, this impact is felt through the steering wheel, seats, and the floor. This kind of impact occurs when driving over a gap or a bump in the road.
1.Main causes
Envelopment characteristics of tires
Worn suspension components (bushings, shock absorbers, etc.)
2.Mechanics of noise and vibration development
(1)A longitudinal impact is applied to the tire, causing a sectional deformation, when the vehicle runs over a bump or a gap in the road.
(2)This impact is further transmitted to the suspension arms. The suspension bushings are therefore compressed, transmitting the abrupt noise or vibration to the vehicle body.
Tire characteristics, and the shape and spring constants of the suspension arm bushings have the most decisive influences in the development of harshness.
1.Tire characteristics
(1)Tires with low vibration-transmitting properties are able to absorb vibrations: Usually these are soft tires.
(2)A tire with good envelopment characteristics:
Since radial tires have stiff belts in their treads, their envelopment characteristics are inferior to those of bias tires.
The force of the impact created at the time the tire runs over a bump or a gap on the road is a combination of the tire’s vibration-transmitting and envelopment characteristics. For this reason, when a radial tire, which possesses inferior envelopment characteristics in comparison to bias tires, rolls over a gap or a bump in the road, it readily transmits the impact to the suspension at low vehicle speeds. But at higher speeds, the softer sidewalls of the radial tires are capable of absorbing such impacts. 2.Suspension arm bushings
The longitudinal slits in the suspension arm bushings are designed to soften the fore-and- aft impacts applied to the vehicle by allowing the bushings to flex. The inter-rings provide rigidity in the transverse direction.

Here, we are concerned not so much with vehicle vibrations, but with the bouncing and swaying of the entire vehicle i.e., the transmission of every unevenness of the road surface to the body. These symptoms appear when the vehicle runs over bumps or uneven road surfaces at particular speeds.
1.Main causes
Bumpy or uneven road surface
Tire vibration
Suspension vibration
2.The development of discomfort
(1)The vehicle body sways when driving over bumps or uneven road surface. These oscillations are transmitted to the suspension.
(2)The oscillations in the suspension cause the vehicle body to bounce repeatedly, making it appear to the passengers that the vehicle is swaying.

This type of vibration causes the shift lever to oscillate, usually at relatively high engine rpm. It tends to be more selective about the rpm at which it vibrates. Of the various types of shift lever vibration, if the vibration occurs when the engine is idling, see the section on “Vibration at Idle”.
1.Main causes
Out-of-tune engine.
Imbalance in engine rotating or reciprocating components.
Resonance in the shift lever, or lack of rigidity of the shift lever.
Play between the shift lever and linkage, or worn bushings.
2.Mechanics of vibration development
(1)Front-engine. Rear-wheel drive vehicles
Engine torque fluctuations or imbalances in rotating or reciprocating components generate bending vibrations in the drive train. Moreover, the joint angles or an unbalanced propeller shaft may further amplify the vibrations.
The transmission extension housing is thus vibrated vigorously, generating a vibratory force which is transmitted to the shift lever.
The shift lever oscillates, since it is mounted on the transmission extension housing. Any play in the shift lever further amplifies the vibration.
(2)Front-engine. Front-wheel drive vehicles
An out-of-tune engine runs rough, causes the shift lever assembly to vibrate.