Maybe the fit of the pad driver to the plastic drive is loose. I just received a new pad driver from Challenger. It was just a tad loose, which caused it both to keep falling off, as well as causing a lot of noise and vibration when in use. I added 2 layers of black plastic electrical tape to each of the three sections of the plastic driver. Now it fits nice & tight, and just purrs.
to Marc:
try e-mailing the Watsons at challengerop@gmail.com.
If it's a technical matter, I'm sure from past experience that Lee will be in touch with you promptly.