"if the average CPU temp over the last 3m is below 35C, stop the front fans, otherwise spin them at 500rpm until the average CPU temp for the last 3m goes over 45C, at which point ramp up to 1200rpm ...

So I built out a new box some months ago with the help of the fine Ars folks and it's been running great until recently.. for some reason the fans seem to randomly ramp up high for a few seconds then ...