Thumping noise by the belt pulleys

The only way I can describe the noise I am hearing is that is sounds like a belt has broken and is flapping against the firewall. I ckd with a flashlight and the sound is coming from that area with the pulleys. The van is running fine. Temp is fine. It is driving me crazy what this noise is. Anybody ever have this problem? 1999 oldsmobile silhouette.

Remove the belt and check to make sure its not missing a section of rib. While the belts off rotate all the accessories and make sure they are smooth.

Found the problem. A harness of some sort was laying up against the backside of the belt. Took a metal wire and move the harness over.