Has the Dryer Had a Period of Non-Use?
If the dryer hasn't been used for a while, there may be a light thumping sound during the first few minutes of operation. When the dryer is not used, the drum support rollers can flatten where the rollers and drum touch. The flat spot causes the thumping sound. Once the dryer runs for a period of time, the rollers will regain their shape and the sound should stop.