Product review (5 stars): Dryer Squealing?  Try This First!
If your dryer is making an unbearable squealing noise, and you don't think it could possibly be the Idler Pulley, spend the 10 bucks to give it a try.  It's a 5 minute fix, and amazingly, it worked.  Sure beats plunking down $500 (or more) on a new dryer, or $100 to have somebody else diagnose and repair it.

We had already replaced the Rear Bearing, which is also inexpensive, but much more time consuming and difficult to replace (it involves taking the entire dryer apart, whereas this is just removing the back access panel).