First, you need to determine where the noise is coming from. The easiest way is to stick a regular plastic straw into your ear and bend down and try to find the source of the noise. Once you've located the source, try these steps:

1) Spray a few light "spritzes" of WD-40 (available at any hardware store or at most supermarkets) into the circular grille of the power supply fan on the back of the computer (if you have a circular grille). This is best done with the computer turned OFF.

2) Turn the computer back ON and see if you still hear the grinding noise.

3) If you still hear the noise, then check your CD-ROM Drive to see if you have a CD in it. If the CD is slightly warped or not perfectly round, it may vibrate in the CD-ROM drive. The CD-ROM drive is usually accessed when the computer is first turned ON, but then it will stop spinning after a while.

4) If you still hear the noise, then you may have a bad CPU fan that attaches on top of your Pentium chip (or Pentium-clone chip) to keep it cool. This will necessitate opening the case and spraying the CPU fan with WD-40 or replacing the CPU fan. This is also best done with the computer turned OFF. If the CPU fan needs to be replaced, it is safer to ask your local computer consultant or computer repair shop to do this for you instead of trying to do this yourself.

5) There is also a small fan on the video card. You can try spraying some WD-40 on this fan. If this doesn't work, since it's pretty difficult to remove this fan and install another fan of the right type, it might be easier to just replace the video card with the same or similar model. This is best done, of course, by a professional.