As I said:
-It is definately NOT a fan
-It is definately NOT my monitor (it's LCD anyway and the noise continues when it's off)
-The sound definately comes from within the computer case
-It may be a coincidence, but it seems to have started when I got my humidifier, but I also added a hard drive due to my recent crash

I'm pretty sure it's a coil as mentioned above but I haven't confirmed this yet.