I have a '94 prizm that makes a faint grinding-type noise with a slight vibration starting at 60 mph that gets louder the faster I go. It does not change with acceleration or braking. I can't hear it at lower speeds. It is a 4spd auto without a differential. I can't tell whether it comes from the back or front. Anyone have an idea what this could be?

An update--I jacked up the car today and spun each wheel. I did not feel any play or grinding or stuck brakes, etc. I still don't know what the problem is. Any suggestions would be greatly appreciated.
Thankyou in advance.