Test 1
To clearly determine the nature of the noise, drive the car on the highway at approximately 20 km/h. Then gradually increase the speed to 90 km/h, listening simultaneously to different types of noise and noting the speed at which they appear and disappear.
Release the accelerator and reduce engine speed without braking. During deceleration, monitor the change in 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 to a stop. Monitor the noise at different deceleration speeds. Be careful and gentle when turning off the ignition. Do not turn the key more than necessary to turn off the ignition, as turning the key further to the "Park" position may trigger the anti-theft device.
The noise noted during this test, which is consistent with that noted during the first test, does not originate from the final drive gears, since they cannot produce noise during the second test. On the contrary, the noise noted during the first test and not repeated during the second test may originate from the reduction gears or the bearings of the pinion or differential.
Test 3
With the vehicle stationary and braked, start 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 first 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 this, 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 units, such as the suspension or body.