@Nardi, is the sound "definitely" coming from the CPU? If so, then it would clearly have to be your CPU air cooler. Any chance that you could make a recording of the sound and post on YouTube or somewhere that you can post a link to? Accepting your assumption the the sound is from the CPU, then perhaps you need to replace the CPU heatsink fan. Personally, I think it's worth reseating the CPU cooler *IF* you have (1) thermal CPU compound (e.g. Arctic Silver 5) and (2) isopropyl alcohol, to clean off the old compound before you apply the new stuff. Since this problem began after you replaced your 500 watt Myo, I'm thinking that the CPU cooler got jostled and needs to be reseated or replaced.
By the way, what is your CPU? AMD FX83xx ? And what CPU cooler are you using? By the way, what country do you live in?