I started this computer with a Corsair VX450. It was very quiet when the system was stressed (100+ watts), but at idle and especially when the system was suspended it squealed like a tiny banshee. So I tossed it in a box and got an Enermax Modu 82+ (500 watt version). It was very quiet until I was forced into a video card upgrade that's taken my power consumption at load from about 140 watts to more than 220. At around 200 watts the Enermax starts squealing just like the Corsair. So... I'm lost on how to address this.

Normally I'd ask for suggestions for a better power supply, but I've already purchased what I had thought were two high quality, quiet, units. Can someone recommend one that really won't squeal?

People here complain about your problem from time to time. It seems there is no PSU brand or model that is immune to it. Changing components seems to often fix it.

It may be that changing PSUs, even within the same model number may fix it. I myself have never run into this problem... but others in this forum have.