After installing a new belt the drum turned properly for a few minutes then the idler arm and idler wheel shifted toward the motor pulley, made contact and began making a thumping sound. I checked the belt position and confirmed it was not twisted. I recently renewed multiple parts including rear bulkhead felt gasket, front glides, and rear drum rollers. If I manually pull the idler arm toward the spring about 1/2'' the noise will stop and it runs smoothly. I thought perhaps I had the wrong belt so I pulled it off and compared it to the old one.

Appears identical but since the old belt had not caused a problem I switched to the old one. This solved the thumping but I notice the idler wheel and the motor pulley are still so close that the pulley is scraping the idler wheel. If I reset the belt and idler position and turn on the motor I can watch as the idler arm moves slightly under spring tension but keeps 1/2" distance from the pulley then after a minute or so swings toward the pulley, makes contact, and remains in that position. Any suggestion as to the cause? Weak spring? New belt too tight? Thanks