Product review (5 stars): If your dryer makes noise with heat on but not with heat off, this might be the problem
Wish I had figured out this was the problem with my dryer sooner. After going through the drum bearing, belt, and motor, it took some detective work to figure out these things go bad. At least, it seems, when they get hot, they get out of round and start making noise. If your dryer makes noise after a few minutes with heat on, but doesn't when you run it without heat, this is probably the problem. I replaced it and it's been super quiet ever since! I wish it was a little less expensive, but whatever. It's cheaper than a new dryer!