As far as my own personal experience (three different 522s, several months of use, pretty close scrutiny) I've never seen the fan come on out of the blue. It only does it for a few short moments when the unit reboots. So I would agree that you might be having heat/airflow issues. But like Simon said; at least it sounds like it's working if your stereo is what's causing the extra heat. When I first got mine I thought the fan was broken because it never came on except for reboots. I was thinking of it like a computer where the fan always runs.
I've had a 522 since Feb or March. When we first got it, we'd be in the other room eating dinner or something and it would wind up, either the HDD or the fan. We thought, that's odd, we're not using it....
It's now August and I'm confident the 522 still "winds up" ..... but we don't hear it anymore.