My dryer is making a loud squeaking noise only when turned on. If you spin the drum by hand it does not squeak. Could it be the rear suport roller or the motor bearings?

You need to take the dryer apart and clean it and grease the bearings with white lithium grease. Look at these links:
http://www.youtube.com/watch?v=MMP7BW1lLs4 to see how. Also: http://www.youtube.com/watch?v=iBiRw-KElC4 ; http://www.youtube.com/watch?v=yFTZGgVisCI
http://www.electrical-forensics.com/Dryer/ElectricClothesDryers.html
It's really easy to do, just get dryer out in garage where you have plenty of space and wash out all parts that handle air, after removing them. Make sure to clean blower wheel and vacuum out motor. Lint deposits build up like concrete and will cause a fire. Once you get it clean you can grease the bearings, and I advise replacing the belt, too. DrTom4444