am1h-itx fan cpu fan issue

I've had the am1h-itx for a couple of months now, quite happy with it. I built an htpc with it:

athlon 5350

4gb corsair ram

128 crucial ssd

itx box with 90w psu

win 8.1

The fan profile is set to silent, which works great, browsing & video playback don't push it enough to hit higher rpms and the pc is almost completely silent. OS power profile is set to sleep mode after 30 minutes.

The issue I'm having is, that after a few hours of sleep mode, the fan start getting louder and louder. For example, after an evening movie, I manually turn the pc off into sleep mode. At that point the fan turns off, video output from the board is gone (no signal warning on screen) and the pc seems to properly sleep. If I press a keybard key, the pc wakes up and the desktop is shown again. All good. If I then leave the pc in sleep mode and come back after a few hours, the fan is spinning like crazy. If I turn the tv on, the "no signal" is still there. If i press a key, fan immediately stops (or drops rpm to silent) and the desktop is shown again.

I've got the latest drivers and I'm using the latest bios (1.5). Any ideas what might be wrong or what to try?

Thanks