Pounding Noise when printing

Out of the blue my 4250 started making a grinding/pounding noise when it started up or when printing. I tried changing the toner, then did a lot of reading about possible causes. I purchased a new swing plate assembly and changed out the gear that seems to cause 99% of these noises. I must be a 1%er, as the noise continues. I happen to have a 2nd 4250, and swapped out the fuser, but still the noise persists. The noise is definitely at the rear of the printer. Where else do I look? Thanks.