308 HDI humming sound

Hello, I took the vehicle to both authorized and private service today. The problem with my car is that when I use the car, especially after passing 120 km, I get a humming sound from a point we cannot determine and the sound continues until the car drops to 50-60 km. They said that after the authorized service controls, the sound came from the gearbox and should be opened and looked. The reason is that the transmission balls are likely to be scattered. In the first place, they said that the problem would be about 1600 TL. However, when I went to the private service, I learned that the problem was probably caused by tires. He said the same thing in rot balancer. The petlas starmax speed tire in the vehicle will be replaced on Monday with 4 tires, the wheel will be replaced for trial purposes. What I do not understand is how logical it is for a non-worn gearbox at 110,000 km to pay so much. I am waiting for your valuable comments.

Yes, since the transmissions are zero, the bearings are out of order.

So what should we follow to avoid being deceived? They wrote 950 TL of workmanship and the same sound even when the gear is idle