Last night, the optical drive fan decided to run at 100%. I didn't think anything of it at first, it's done it before but would eventually die back down. But this time is not the same, so I did some investigating, first by installing MacFan and the first thing I noticed was that the ODD fan was reading 0 rpm. I've already been itching to opening up the iMac and I was planning to do it when I install a SSD. Opened her up (very easy), and was going to disconnect the fan completely but thought, if I wiggle it, it'll probably work. So, wiggled it, pushed on it, and stared at it for a few minutes, put everything back together, started her up and silence. Fixed it (1148 rpm), so if anybody run into this fan gone wild, now you know.