Diagnosis
A worn out drum support roller or a foreign object (like a coin or bra wire) caught between the drum and the rear bulkhead.
How to Fix It
Unplug the dryer. Remove the front panel (usually two screws at the top of the opening, then slide the top forward). Manually rotate the drum and listen/feel for grinding. Inspect the rear drum glides and the support rollers at the back for excessive wear, cracks, or a missing bearing. Replace any worn rollers.
If That Didn't Work
Check the drive belt for damage or if it has jumped off the idler pulley. A frayed belt slapping the inside of the cabinet can make a rhythmic thump.
Heads Up
Running it with a damaged roller can grind into the dryer tub, turning a $30 part fix into a $200+ tub replacement. Stop using it until diagnosed.