Jeff:
2 things comes to mind.
1) Failing motor
2) Worn/dirty capstan bearing
Dismantle tape deck and remove the main belt coupling the motor to the flywheel. Turn the deck on.. if it still "growls" its the motor. (B&O has a replacement you can buy but I am told it is not as nice as the original one).
If it is a worn/dirty capstan bearing, it may need replacement or lubrication.
Those are my guesses... Martin may know better.
Derek