It sounds like the fan in the psu may be bad ( the buzz you're hearing). PSU's are pretty cheap but you get what you pay for.. check
www.pricewatch.com for prices. You'll find that you can get a power supply for under 20 bucks.
I'd also check that the cpu fan is turning as well.
If the fans look ok, strip the machine to a bare bones configuration (boot disk, ps, video card, mem, keyboard/mouse) and try to isolate which device may be causing the problem. If you can't change the symptoms, psu is the likely culprit. (Don't over look the mem sticks as a possibility. If you have more than one, try one at a time or try swapping them around).
If you're not comfortable working inside the case, I suggest you find someone who is to do these things.
Good luck.