This is a noisy Duet style Whirlpool front loading washer. It begins to make a knocking sound as the unit begins to wind up on the spin cycle. It is not the normal rumbling sound of a bad bearing. To my ear it sounds like a loose pulley wheel – but I’ve tightened the pulley nut as tight as I can, and the sound is still the same. When I look at the edge of the pulley, I notice that it is not completely round – it was a wobble to it. So I will try replacing the pulley to see if that fixes the sound. If not, I guess it will be the rear drum bearing.
Update: this one did end up being a rear drum bearing. Usually the rear bearing makes a rumbling or grinding noise – this noise was different than the norm. But it is fixed.