Intermittent noise from case?

I've had my tower for a little over a year now and tonight I noticed an intermittent noise coming from the case as it is running (a slight sort of screech, like rubber tires on the road maybe?). As I type this I don't hear it, and it seems not too bad, but I am concerned obviously for what it is. I have an intuitive guess that it may be an older hard drive failing/dying.

Does anyone have any ideas/any way for me to figure it out without moving it/adjusting the case? I'd rather not risk it here at school for the time being.

I also don't have transportation so I can't take it anywhere if need be.

Yes, full specs please. But another potential is a dying fan I suppose. Backup your data ASAP to be safe regardless.

Open up the box and listen for the noise.