Thanks for those. I do normally leave it running, on the less wear basis, although I have HDs set to turn off after 20mins. in the energy saver bit (!).
It's due for a vac - usually a couple of times a year, and while I'm at it I usually give everything socketed a tap with a screwdriver handle.
It's still running, so I will have to wait to next bootup and see if it does it again.
Oh, it's HD temp that's showing.