DIAGNOSIS That rhythmic thumping is almost always a foreign object—like a coin, button, or hairpin—loose in the drum or caught in the blower wheel area. It gets tossed around during the spin.
FIX Unplug the dryer. 1) Remove the lint filter and check its housing cavity with a flashlight; small items fall in there all the time. Use needle-nose pliers to fish anything out. 2) Manually spin the drum by hand and listen for the object rattling inside. Tilt the dryer back (get help, it's heavy) and look under the drum through the lower access panel if possible. 3) For a more thorough search, you can remove the front panel (requires a screwdriver) to access the front of the drum seal.
IF THAT DIDN'T WORK The noise could be from a worn drum support roller or glider. If it's more of a squealing or grinding than a thump, and happens even with an empty drum, one of these plastic or rubber parts is likely worn out and needs replacement.
HEADS UP Don't ignore a persistent banging. A hard object can eventually crack the baffles (the lifters inside the drum) or damage the drum finish. Always check pockets before loading, and consider using a laundry bag for small, delicate items.