Lexmark All-In-One Printer Scanner Problem?

Hi, I have a Lexmark X4875 and I have a problem.
When I switch it on the scanner head moves across the bed but when it gets to the end I get a knocking noise. It's like the scan head doesn't know it's got to the end and keeps trying.
This printer is being used as a stand-alone so the problem has nothing to do with my PC or any software.
I'm happy to go inside the printer/scanner if that’s required.

Blackolives
February 2010