humming sound from the front

[signal hoss <simails@xxxxxxxxx>]

My 1991 318is has a humming sound from the front
when driving, and it disappeared when car stopped.
The humming sound soulds like base in sterio and
seems in sync with rotation of tires.
I took the car to the dealer and they said it was 
timing belt and replaced it. But the sound still
persists.

Does any one have idea what happened and how to fix
it by myself ?

Thx.
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