High pitched sound coming from the front of my computer by the power button, what is it?

There is a faint high pitch sound coming from the front of my computer by the power button. Sometimes I can't hear it sometimes I can. I put my ear but the powerbutton but it didn't sound as pronounced as it does standing away from it. The sound is comparable to like when you turn a tv set on without the cable and you hear the faint siren sound. This isnt as loud as that at times but sometimes it is and i hear it pretty well. It started last week then i think it sent away but last night it came back and its been doing it all day. Whats going on?