Humming/buzzing sound traced to Extension Box?

Recently I noticed a new humming/buzzing sound occurring only when the car is moving and audible at low speed. (This happened very shortly after a service which included new brakes, but I think that's a coincidence-It doesn't sound like any brake noise I've ever heard).

My mechanic test drove the car and put it on lift to try trace the noise. He determined it was likely a "rubbing sound" coming from the extension box in the transmission and referred me to a transmission shop to further investigate. He said it's not urgent.

My questions are:
1. Any ideas of what this might be?
2. Is it reasonable to just wait and see if it gets worse before doing anything further?
3. If it does need an expensive transmission repair, how would I determine if it's worth it?

Thanks for reading my question!