Grinding sound from gear box

Hi, I have a terrible crunching sound coming from what I would assume is the gear box in my 2013 Q7 3.0 auto 8speed with 70k on the clock. The sound doesn’t start until I select drive or reverse. On pulling away the noise can take a while to start but when it does it’s a horrible crunching/grinding noise. I recently changed my front right tyre and found out these need to be done in pairs. The difference between tyre is, Front left 5mm Front right 7mm. I’ve been told by 3 mechanics that diff wind up won’t be the issue due to 2mm. I will try and up load a video if I can. Has anyone else experienced similar issues? If so what was the issue and how was it repaired? Thanks for any help.