How can I quiet down a clothes dryer?
I think your earlier info about main bearing or roller guide wheels sounds good. Let me recommend RepairClinic.com, they have a section there on repair help, common problems, and specifically noisy dryer. They typically recommend replacing guide wheels, etc, for noise issues. Let me recommend AGAINST soaping the belt. That sounds like a bad idea. If the belt is what transfers motor power to the drum, to spin the drum, soaping it (which would reduce the friction) sounds very bad indeed. 
Hope this helps.