This is one of the features of some gear box models. The characteristic hum is lost after warm-up and at higher speeds. A few thousand kilometers of mileage (usually after the first TVE), the boxes are titillas, and the hum is completely lost.
It should be noted that it is often difficult to include some of the gears at the unheated checkpoint. This shortcoming has also been addressed by itself over time.
But if the time of the box is not lost, you should try to replace the transmissive oil. If the problem is not solved, then I will have to change the bearings of the primary PPC-in this case it is better to turn to the service.