If you're CERTAIN it's the PSU, and the squeal is electronic and is definately affected by a change in current draw, then what you're hearing is accoustic leakage from the switching transistors in the PSU.
Monitor high voltage power supplies can also do this. I have a monitor that "sings" at a very high frequency when there's a lot of white on the screen.
If you're young or (like me) have excellent hearing (can hear beyond 17KHz) it can be quite annoying. Moreover your friends or office workers will think you're insane because they can't hear it. Few people over 35 can hear anything beyond 15K (and even if you're young but listen to a LOT of loud music, you've probably blown your high frequency hearing as well).
I used to be a sound engineer and test my hearing regularly and happy to report that as of last year I could still hear over 21K.
Nothing you can do except replace the PSU. Sell the old one to someone who can't hear it
