Try this. Grab a hammer or something solid and give the power supply a gentle tap from the back of the PC. You may have to smack it a few times. If it's the PS Fan, it'll quiet down when you smack it.
Chances are you can get a power supply fan for $50, depending on if you screwed up and bought some crappy Micron or something (not all those manufacturers are using standard sized parts). I run my PC constant, never turned off, so I replace my PS pretty regularly as most PS Fans are hardwired in and I'm smart enough not to go taking it apart and running for my solder iron.
It's not a steady click - click - click - click sound so it's probably not your HDD taking a shit. Start with the PS.