While my PC is on, it keeps making a sound like it's attempting to power down. This sound is heard whenever I power down the computer. At first I thought it was my master drive which did need replacing. I successfully replaced it and now I still hear this sound like the PC is powering down or the something is about to shut off. Any ideas what it could be? This is not normal. Is there a diagnostic utility I could run to see if any of my hardward is going bad? Thx.

The sound you hear may be your optical drive(s) spinning. Windows spins the optical drives at seemingly random times when they have a disk in them, often without lighting up the drive's led.
Remove any CDs or DVDs you have in drives, and try your computer for a while to see if the sound has gone away.

If it ain't that, you're probably hearing a noisy variable speed fan - the one for the cpu, or the power supply.
.......

Test the hard drive you replaced.
There's probably nothing wrong with it.
Or test your present hard drive.

Check your hard drive with the manufacturer's diagnostics.
See the latter part of response 1 in this:
http://www.computing.net/windows95/...

(thanks to Dan Penny for this link:)
Hard Drive Diagnostics Tools and Utilities
http://www.tacktech.com/display.cfm...

If you don't have a floppy drive, you can get a CD image diagnostic utility from most hard drive manufacturer's web sites, but obviously you would need to make a burned CD, preferably a CD-R for best compatibilty, on another computer if you need to.