Trial 1
To clearly determine the nature of the noise, develop the car on the highway at a speed of approximately 20-30 km / h. Then gradually increase it up to 90 km/h, listening at the same time to different types of noise and noticing the speed at which they appear and disappear. Release the throttle pedal and reduce engine speed without braking. During deceleration, watch the noise change, as well as the moment when the noise increases. Normally, the noise comes and goes at the same speeds both when accelerating and when decelerating.
Trial 2
Accelerate the vehicle to 100 km/h, place the gear lever in neutral, turn off the ignition and let the vehicle roll freely to a stop. At the same time, the nature of the noise is monitored at various deceleration rates. When turning off the ignition, be careful, as the anti-theft device may work.
The noise noticed during this test and consistent with that noticed in the 1st test does not come from the final drive gears, since they cannot make noise when unloaded. Conversely, noise noted on the 1st test and not repeated on the 2nd may come from the gears of the gearbox or the bearings of the drive gear or differential.
Trial 3
With the car stationary and braked, turn on the engine and, gradually increasing the speed of its crankshaft, compare the resulting noises with those seen in previous tests. Noises similar to those from the 1st test indicate that they do not come from the gearbox, but are caused by other components.
Trial 4
Noises found in the 1st test and not repeated in subsequent tests come from the gearbox. To confirm, lift the rear wheels, start the engine and shift to IV gear. In this case, you can make sure that the noise really comes from the gearbox, and not from other components, such as suspension or body.