Suddenly, T5400 fans are on full, constantly

Until the other day, everything seemed normal. One day, I turned on the T5400 and the front fans just started speeding up during boot. Were at full (or what sounds like 100%) by the time Windows loaded and never came back down. No reported temperature is high. Right now, the highest temp for a core is 42C and for RAM 45C.

No hardware changes for months.

This only ever happened before when I would power up without the side panel on. Haven't done that for almost a year (during a 2nd cpu install). I assumed back then that it was because the case was open. If I held in the 'intrusion button', the fans didn't kick on to 100%.

according to FanSpeed, 3 fans are hovering around 3k rpm, one is around 1.5k (CPU-Z's HWMonitor doesn't apparently see the fans).

Any thoughts why? The noise is very annoying.

dual quad Xeon 2.83
16GB RAM
500GB HD
GTX260
9500GT