Turns out the problem wasn't the fan at all, but something in the fan mounting. It's temporarily fixed by removing the fan from its mounting bracket and just sitting it in the case. They're ordered a whole new case, and will come out to install it (more correctly, to move everything from the old case to the new one) after I get back from Madrid.
Now, that's weird. Still, it's quiet once again, and nothing's complaining (and, in fact, the drive temperature has dropped from 115 to 85 degrees F), I can live with it until they come out and fix it right.