Test 1
To clearly determine the nature of the noise, accelerate the car on the highway to approximately 20-30 km/h. Then gradually increase it to 90 km/h, listening simultaneously to different types of noise and noting the speed at which they appear and disappear. Release the throttle pedal and, without braking, reduce the engine speed. During deceleration, watch for changes in the noise, as well as the moment when the noise increases. Usually, the noise appears and disappears at the same speeds both during acceleration and deceleration.
Test 2
Accelerate the car to 100 km/h, put the gearshift lever in neutral, turn off the ignition and let the car roll freely until it stops. At the same time, monitor the nature of the noise at different deceleration speeds. When turning off the ignition, be careful, as the anti-theft device may be activated.
The noise noted during this test and consistent with that noted in the 1st test does not originate from the final drive gears, since they cannot produce noise without a load. On the contrary, the noise noted in the 1st test and not repeated in the 2nd test may originate from the reduction gears or the bearings of the pinion or differential.
Test 3
With the vehicle stationary and braked, turn on the engine and, gradually increasing the rotation speed of its crankshaft, compare the noises that arise with those noted in the previous tests. Noises similar to those that arise during the 1st test indicate that they do not come from the gearbox, but are caused by other units.
Test 4
The noises detected during the first test and not repeated during subsequent tests come from the gearbox. To confirm, lift the rear wheels, start the engine and engage the fourth gear. This will make sure that the noises really come from the gearbox and not from other components, such as the suspension or body.