Three times now, over the last two months (at least three times that I've been around to notice it), my iMac's fan has suddenly spun up from near-silent to full-blast-I'm-about-to-take-off-from-your-desk. It then gradually (VERY gradually) winds down to normal, near-silent speeds again over the course of about a minute and a half. I've got iStat, and there's only minimal CPU usage and there's no temp spikes when this happens. This time I was just browsing the web; last time it happened, I wasn't even using the computer. Any ideas to fix this? Anyone else have this happen?