Why is my dryer making unusual thumping or vibrating sounds?

A thumping sound during the first few minutes is normal if the dryer hasn't been used for a while. Vibration can be caused by clothing balled up in the drum, all four leveling legs not being installed, or the dryer not being level front-to-back and side-to-side. Check the drum edges for small objects like coins or buttons that may have fallen from pockets. (Page 9)