skinlessfoot

I just bought an '84 subaru wagon with AT, 4wd. When a go faster than 20-25 mph it starts to make a loud vibrating sound (kinda like turning on a microwave, but way louder) from the front of the car. It makes this sound regardless of what gear it is in (1,2,D) or in neutral. I was thinking that it could be coming from the transaxle, the gear oil looked like it needed a change but other than that, I don't know what the problem could be. Wheel bearings, or bearings somewhere else in the driveline? Any advice appericated. Drew