Dryer thumping at startup then gets quiet

To describe the thumping - it's a loud, thump, thump, slight pause, thump, thump, slight pause, etc. After the drum heats up the sound quiets down.

What I've done:

Replaced the rear rollers - didn't look like they need it but I had the machine torn apart. The rollers spin freely on the axels but I tried to replace them anyway. I couldn't get them off--GRRR!

Completely cleaned the air outtake to ensure there isn't something behind it.

I know this sounds like a flat spot on the rollers but the machine was quiet for years and then started making this noise. I'm trying everything to not buy a new machine.

Any suggestions?