I recently purchased two different cases and each included a P4 ready power supply. When in use, both "Generic" power supplies cause AM radio interference (high-pitch squeal). I have not tried using them with P4 MBs, hence the P4 connectors are not being used. I know you get what you pay for, and few people listen to AM radio, but both PSs have the Fcc symbol and they should not do this. So why do they? The PSs are "TurboLink CWT-320ATX" and "Elevation LPF2N".
P.S. My Enermax and Antec P4 ready PSs do not cause this problem.
I wouldn't rule out fake FCC or UL numbers or changes made from the design submitted for testing.
If the noise decreases when you plug the power supply into a good surge filter, one that contains an EMI filter as well as usual MOVs, then the supply may be lacking its own EMI filter and have several empty spaces on its circuit board for the capacitors and chokes that make up this filter.