Is the load knotted or balled up?
Another form of thumping can occur when the load is twisted or balled up. Laundry items with large amounts of fabric, such as sheets, curtains, tablecloths and bedspreads, can roll into a ball in the dryer. This ball of fabric may thump and may cause the dryer to vibrate. To reduce this, add these large items loosely into the dryer. Remove any bunching that may have occurred during washing.
Were you drying tennis shoes?
Tumbling tennis shoes in the dryer can make the dryer shake or vibrate. Use a dry rack when drying tennis shoes.
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.