It was the 'Sleep' button on my keyboard, and it was accidental honest... anyway the system glitched and refused to wake up so I had to reboot. The BIOS decided it was Level 1 Diagnostic time and reset to its defaults.
Fine, except - noisy fans! I had to venture into the BIOS to switch variable speed back on. I hate doing this, even though BIOSes have help screens these days they are in, well this being ASUS it's Chinese English.
So far so good, I tweaked the chassis fan speed down one step from 'whine' to 'hum'. ASUS have a free program called PC Probe that displays this kind of stuff and can set off alarms if things go wrong. Next I'll try going down to Super Quiet Mode!