Grinding or oscillation noise at approx. 1500 rpm

Hello together, For some time now I have a grinding or oscillating noise at my Phaeton. It always comes at a speed of about 1500 rpm and gets faster and faster depending on the gear height. 2 Vw service partners have already failed in the troubleshooting. Motor and wheels can be excluded however. The opinions go in the direction of the gearbox but you don’t want to decide. For me it sounds like it comes from some bearing. It’s also amazing that it’s clearer is when the car is warm. Does anyone know or have ever had such a problem? It is a Phaeton from 4/2015 with the 3.0 TDI and of course automatic transmission. I am grateful for all the ideas!