My PC is making bizarre noises

Hey guys, I'm in desperate need of help, my PC is making really weird noises. I'm not sure what it is, but even though it sounds like the fan, it's not the fan. When I turn on my computer, I get the normal sound of the fan going on, then a couple of seconds later I hear this weird sound. Me and my brother opened up my pc but we couldn't find out where exactly the noise was coming from. We unplugged the dvd drive and the hard drive but we still got the same noise. And according to my brother, it's not the GPU either, since when he turned off the pc the fan of the GPU went off before the sound dissapeared. It's really hard to see things in my computer since I can only open one side, and the wires are blocking a lot of things, so I'm kind of wondering what you guys think it is and if there is anything I can do to stop it.

Just so you know, the noise had started in June for a few days, but then it stopped. During most of the summer it was unused since I was on vacation, but, now, a few weeks after I get back and used my computer (with no problems), it's starting again. Usually, if I hit the side of the computer with the tip of my two fingers hard enough several times, the sound could stop, sometimes it didn't.

Oh, and after manually restarting my computer a few times prior to opening it, my windows kind of bugged and vista apparently repaired the bugged files, it worked fine after.

I bought my HP PC premade with a Q6600 3gigs of ram and a 8500GT, and replaced the GPU with a 8800GT.

Thank you in advance.

Edit: Oh, and the inside of my pc is surprisingly clean.

without knowing where the noise is coming from it makes it difficult o tell you wot to do.

first of all one of the most common causes of "a wierd fan-like noise" in a computer with a lot of wires is a wire is just catching one of the fans, so when the fans turns its hitting the wire. this may explain why when you hit it it stops. so first try moving ll the wires away from your fans and see if thats works.

if not its the tenious task of taking all the components out of your computer one at a tim until you find the one thats making the noise!

Some people's motherboards are noisy, like mine.

If you have fans positioned poorly or it could be clipping a small wire.

Your motherboard could be loose off the bolt mounts and it could be rattling - which can make all different noises.

Your CPU heatsink/cooler might not be screwed in securely and it's coming loose so it rattles a bit and vibrates.