I, too, got bothered by fan noise at night, and my server was being hacked.
Well. Hacked as in someone banging their password list against an SSH server that only accepts public key logins, at maximum speed with tens of simultaneous connections, pegging the CPU (and I was running junk hardware, I found that 800MHz Intel Coppermine CPU literally from a mixed waste trash bin). Usually the scripts doing this had the decency to keep the attempts to something like 1 per second which would be unnoticeable.
Well. Hacked as in someone banging their password list against an SSH server that only accepts public key logins, at maximum speed with tens of simultaneous connections, pegging the CPU (and I was running junk hardware, I found that 800MHz Intel Coppermine CPU literally from a mixed waste trash bin). Usually the scripts doing this had the decency to keep the attempts to something like 1 per second which would be unnoticeable.