I just had a nice experience that I thought I'd share. For the last month or so, my QMS-PS 410 laser printer has been making these groaning sounds at two separate spots in the paper path. Nothing seemed to be wrong, but I found the noises rather alarming, so I finally called QMS tech support. They weren't in, but Joseph from QMS called back shortly and said that QMS had noticed similar problems with Hammermill paper, and what sort of paper did I use? I told him about the Finch Laser Opaque (24 lb) that I'd been using for the last few months, and he suggested trying some plain old copier paper to see if the sounds went away. I tried some different paper, the sounds stopped, and I thought the case was closed. Much to my surprise, Joseph called me back today to see if the different paper had worked or if QMS should ship me a little cork piece that he said was probably the cause of the noise. Now that's what I call customer service!
For those of you with similar printers using that Canon engine (the HP IIP and IIIP and the Apple Personal LaserWriter NT, SC, and LS), keep the solution in mind. I have the impression that the problem is engine-specific, and didn't just affect the QMS-PS 410 (an otherwise very nice printer that I'm quite fond of).