;-)
My educated guess is to go first with the power supply or a fan. They start doing exactly what you wrote when they get old and they need replacing.
Have ya blown out the power supply and other fans with compressed air? You can tell Windoze to shut down the HD's after 5 mins. under the power options tab, and then when they spin down see if you hear that sound. Should be easy enough to isolate.
Can you open the case and then restart it?
If you can, get your ear close to the power supply and close to the hard drive. You'll know then.
I'm betting that if you unplug the computer and take a shopvac to the power supply fan vent, you might well solve the prob.
If you still can't tell which fan or drive is making the noise, use a screwdriver as a sound conductor (hopefully one that's at least semi-insulated, LOL).
Place the tip near the suspected part and the handle in your ear. Keep moving it from part to part (carefully, on the metal case parts, not the circuitry, DOH) 'til the noise gets loudest.
Could also be the little fan that is stuck on the CPU. The bearings seem to go quite often on them, or the little power supply wires sometimes flop around and hit those blades.
And do blow out any furballs you find.
Although we do need to know....are you logged in?
How hot do the hard drives run?
Run the computer for an hour, then disconnect the power and open the case. If one of drives is scorching hot, that might be your problem. Hard drives will be warm, but should not be extremely hot.
It is more likely that the lubricant in the power supply fan has dried out. Theoretically, you could open up the PS, remove the fan and put a few drops of machine oil on the bearing which is usually under a rubber grommet covered by an information sticker on the fan.
Don't open the power supply if you don't know what you are doing because it has capacitors that can zot you into the next world.
You could unplug the drives and power-up and see if you still have the noise.
Sounds like the ball bearing fan in the power supply needs to be lubricated. I recommend a couple drops of sewing machine oil. If it's a ball-bearing fan, if you pull back the round sticker that is the label of the fan, you'll see a a recessed area silver in color, put the two small drops of oil there and spin the fan and press down on the the fan gently to work the oil down onto the ball-bearings below. Once you work the friction out of the fan by spinning it with your finger, you'll know it's been properly lubricated. Put things back together and all should be well.
I'd just replace the power supply.....they are fairly cheap and ya eliminate one suspect pretty much.....just my opinion . Paging some picky puter pro's.....
Have you checked your CPU fan? It's not a bad idea to replace it...they are cheap and easy to install. Give the guts a good dusting while your at it. (include the power supply too.) If you change out the power supply get something larger than 250 watt...
Solution:
1) Remove it from your desk
2) Tie a 3/8" nylon rope to it
3) Give it to a friend who owns a boat
4) Buy a Mac
Shadowace, you might need to ping others. If memory serves, I'd read where Dell uses a power supply connection that isn't standard, and you might possibly fry the motherboard?
Food for thought. I don't know if things have changed since this was written:
http://www.easydesksoftware.com/news/news21.htm
"....So I did a search on the internet to purchase a Dell power supply, but found that no one on the internet sells one. I also found out that even if you hacked up the computer box to fit a standard ATX power supply, the wiring configuration is completely different, Compounding the pounding, Dell uses a standard ATX plug to attach to the motherboard and at this point the unknowing individual would plug in and fry the motherboard and toast the power supply. So the only choice I had was to go to Dell to get a power supply....."
Power supply's are $9.00, all the way up to the unimaginable price of $49.95. buy a new one and go from there.
That's a small power supply, hope you went for a bigger one..