LaserJet 2100 fan cycling

I've got an LJ 2100M that I bought new about ten years ago. I very soon
put the JetDirect 600n ethernet card in it and it's been taking care of
our low-printing LAN ever since (page count = 19,300 and we're just
coming to the end of our third toner cartridge). All in all, I've been
very happy with this machine.

Until recently, there was quite a noisy computer next to the LJ. Now
that's gone and I can hear things from the printer I never noticed
before. I have no idea when this started but now the fan cycles on and
off even when nothing has been printed for hours (or even days). It
comes on and runs for exactly four minutes, then it stops. Exactly six
minutes later, it comes on and runs for four minutes again. It repeats
this cycle continuously.

Can this be normal?

Thanks in advance for any insight / analysis.

cheers,

Henry