PROBLEM: I have a DELL Dimension 4700. For the past couple of years the fan has gotten louder and louder. It has been sounding like a vacuum cleaner lately. I cleaned the dust out of it, to no avail. I was just about to buy a new computer when I decided to try to fix it again.
ROOT CAUSE: Long story short. The thermal grease between the CPU and the Heat Exchanger (HX) had dried out and was not not conducting heat to the HX. With poor heat transfer, the chip runs hot, and the BIOS cranks-up the fan to try to cool the chip.
FIX: I removed the HX, cleaned away all the old grease, wiped both surfaces clean with isopropyl alcohol, then applied Cool Master Cooling Compound to the interface between the CPU chip and the heat exchanger. Upon re-assembly, the fan noise is very quiet, like when the computer was new. It is a minor miracle! The entire repair took less than 1 hr.
Excellent. I'm surprised about the noise but that certainly seems to have done the trick.
Please post back and let me know if you need more help.