I don't know if anyone has run into this, but here is a post of a problem on these scanners from another forum.

It may be worth looking into if you have a problem.

Name: Scott Hill
Date: September 08, 1999 at 17:56:10
Subject: HP Scanjet 5100C Cxi or Cse

Comment:
If you have experienced the 'grinding' problem and HP has turned you away only to steam with anger. Feel free to email me about this problem. I have a solution for you to try that works temporarily as well as I am organizing a class action lawsuit against them, for they knew 2 months after the products release it has a fault in its design, yet they refused to recall it.
His e-mail address is: jarhead@homemail.com