Belts flapping when AC is on?

This is on a 75 Dodge D100 with a 318.

The belts are all tight and run fine when the engine is idling. As soon as I turn the AC on, the two compressor belts start flapping as if they're loose and it makes an annoying racket. Turn the AC off and they're back to running smooth and tight. They're all tight with the engine off as well. Anyone know whats going on here?