Hard to say. Could definitely be something in the motor, a loose stator, some of the windings coming loose, etc. Could just as well be something with the tire, or possibly a brake pad that has rust or something stuck under it as the reverberation rate increases linearly as speed increases. I'd def take it to a mechanic to at least check out the tires/brakes. If those check out, then I'd lean towards it being the motor.

 

Thanks for the reply, I appreciate it very much!