BELT SLAP!!! Anyone else with this problem?

Might be harmonic vibration. I had one HECK of a time with this with a hang--on A/C setup once, and finally, after repositioning the idler and changing the length of the belt I got it to stop

I'd start by CAREFULLY inspecting the pulleys for any damage, nicks, bends, or even debri

Then try a different belt, a little narrower, a little wider, a little longer, shorter.

If you have the old belt and can exactly replace it (part no, brand, etc) try that.