PC sounds like its ready for takeoff!?

You probably have a speed controlled cooling fan.

more you use the cpu, more it spins fast.the sleeves may be dirty or ruined and there it come the noise.

change the fan over the Cpu and you should be fine again.

Sounds like you should take it outside and clean out the dust.

If that does not work, you can replace the fans with some nice ones that both push a lot of air and are quiet. I reccomend the Panasonic Panaflos (but make sure they are made in Japan).

Hmm...well, my PC has a continuous hum, but my brother who works with computers doesn't seem that concerned. I got mine in 2002, so it is "old" in computerland as well. I do notice sometimes it runs slower than other times. Have you defragmented it? How about condensing some files? Here is something my brother asks me: How much stuff do you have running at one time? Also, I highly recommend getting webroot spysweeper- that helps clean up your system quite a bit. How much memory does your computer have? That also will affect things.

Could possibly be a problem with heating, maybe the fan is failing, or maybe the insides just need to be attacked with compressed air.

It could be overheating but it could also be spyware. I would run an online scan at ewido.net and also download ccleaner at ccleaner.com and the cleaner scan and the issue scan and that should help it run better.