That could be the tub bearings making the noise. With the washer off grab the top of the agitator and try to wiggle the top of it back and forth. Is there much play in the agitator when you do this?
You could also remove the front of the washer and see if it looks like there has been grease flung around inside the cabinet. If there is it's time to replace the tub bearings and seals.
How old is the washer?