I have the same power supply that came with my APEX SQ-328 case (<--look that up on google and see if this is, by chance, your mid-ATX case). The power supply is the Apex Allied AL-C400ATX.
The noise that the PSU makes is from the capacitors and they sound like a high pitch tinitus whining sound. Almost like an old modem sound or cicadas.
I noticed the problem about a year ago, and I took the PSU out and tweaked/glued the capacitors down; the noise went away. This week, the noise started up again due to the changing weather/barometric pressure/humidity.
It's annoying and it tells me that the filter capacitors are probably not smoothing the power. So I bought a Corsair VX450W.