2002 AC Belt flapping

Any idea WHAT is going on here? 2002-- 6.0 motor everytime I get RPM's up minimally with the AC on it sounds exactly like the fan is hitting the fan shroud, low and behold after messing with that for a time I realize the AC belt tensioner seemed like it was not holding under rpm increase making the AC belt flap violently. OK so I replace the tensioner, should be fixed right " wrong" same deal just new belt tensioner, what is going on, does not look like the AC compressor is locking up or anything, by the way, also just noticed I can't seem to shut off AC with teh button, light goes out but compressor remains engaged, don't know if this has anything to do with it. Any help would be appreciated as this is starting to look expensive:screwy:

It is the compressor going bad. Mine did the same thing. I finally had to replace the whole thing.

Make sure the knob is not set to defrost