I just wanted to relate an issue we had in the hopes that it will answer someone else's question:
We recently bought two new Phaser 4600/DT's for our business, about a month apart. After being setup, both units exhibited this problem, right out of the box: A loud "thump-thump-thump-thump" whenever pages were being printed, or when the paper path gears were in motion. The noise sounded a lot like someone was hitting the back of the printer with their fist, at a frequency of about 2Hz.
The problem was due to a faulty fuser, both times. The gearing mechanism inside the fuser unit, where the printer drive gear connects to the fuser, can usually be turned manually by your finger if you take the thing out of the printer (don't burn yourself). It's a bit difficult to turn, but shouldn't hurt your thumb or anything. Well... the Xerox tech showed me our fuser unit. It turned well - except for one spot, where it was very difficult to turn. This non-uniform resistance was causing the drive engine to throw a fit every time the fuser gears hit that spot. The Xerox tech replaced the fuser in both printers, and now they print fine. However - I believe this is sufficient evidence to say that Xerox has a quality-control issue with their 220V fuser units for the Phaser 4600.
Yes, these are 220V fusers (europe). I'm not sure if the same issue would affect 110V units or not... but well, there it is.
Thank you for using the Support Forum. We appreciate you offering this information to other users. I am sorry you experienced this issue but glad that it has now been resolved.