How can I find/fix the squeak in my Bosch WTA3500 dryer?

My Bosch WTA3500/UC 08 developed a squeak a couple of years ago that sounds like metal scraping on metal. We've had several repairmen out and they have replaced the rear bearing twice, and the front felt seal and glides three times. The dryer will start squeaking again just a few months after each repair.
I've pulled the back and one side off and the squeak sounds like it may be comming from the bearing, but the sound travels through all the metal and it's difficult to be sure.
Does anyone have experience with such a problem in these dryers that could offer some advice as to how to proceed?