Proform 540 Treadmill making thumping noises on incline?

I purchased this treadmill Feb 06. First problem ever and I could use some do-it-myself advice (I am somewhat mechanically inclined).

When moving the incline up or down, there is a loud "thump" when it is moving from 1-2-3-, etc. The thump does NOT happen at the half mark ie: 2.5, 3.5, etc...
suggestions?

Proform 540 Interactive Trainer Model PFTL51104.1
holliejustice at hotmail.com