Dell Laserjet 5100CN - Rattling Noise?

I have a user that has reported that their 5100CN is making a rattling noise when they print more than 2 pages? The noise is coming from the top of the printer, and I have looked for anything stuck but it seems ok. It prints out ok, but continues making the noise for another 30 secs after its stopped printing? Always happens if printing more than 2 pages. Printer works fine, its just the noise thats annoying.

Anyone heard of this before? Any idea's of what I should be checking?