So guess what the problem was? Go on, guess.
The last time they fixed it, the tech forgot to plug in one of the fans, so the other one was going nuts trying to take up the slack. Nice.
On the one hand, I feel a little silly that I didn't open it up and check for something so simple, but on the other hand, fuck that, when you get something back from the shop and it's still broken, you're supposed to turn right back around, not try to fix the non-fix yourself.
But, I'm surprised that in my googling I didn't turn up anything like "lmsensors" for the iMac, since that would have made it pretty obvious what was going on.
Ah, blissful fan-free quiet.