Just to be more specific on the hum: It's not the typical 50Hz electrical buzz/hum, it's that refrigerator or compressor hum. The compressor vibrates and, even when it switches off, the compressor does the whole shake thing as is spins down, as if the motor was just switched off in normal...
After we had 2 little power dips (enough to reboot internet router and TV), I heard my fridge click on for 2 seconds, then click off again. Since then, the compressor wont stay on for longer than 2 seconds.
I have checked the the actual compressor starts, and it does but the relay clicks off...