Every once in awhile I have this happen, and it was just recently happening again this morning. The exact details are that I built this PC myself about a year ago and today is probably the 3rd time I had heard these noises. Basically they sound like a HDD being accessed while the PC is off. I tried to Google it and I basically came up with nothing so I figured I would post here and see if anyone has any knowledge of something like this happening.

Do the lights flash at all when you hear noises?

And you did go through the 'proper' shutdown on computer?

Are there any external devices hooked up to the computer?

On a UPS or anything? Maybe a surge protector

Any power surges in home recently?

The HDD access light does not flash at all when I hear this noise. And I always go through proper shutdown.

I mentioned that I built this PC myself, so it's kind of my baby :D , and I always try to take the best care of it that I can.

The reason I THINK it could be the HDD is the sound it makes sounds very simliar to the sound an HDD makes when it is being lightly accessed. It does not make the sound while the PC is turned on, and it was making the sound all day yesterday and this morning while the PC was off. It's not a very loud sound either. I opened the case this morning with the PC still plugged in and it was still making the sound, although I could not tell where it was coming from. I then unplugged it and did a quick check for dust and loose connections and such and also checked the POST code on my MB, which read No problems. When I plugged in back and let it sit for a couple minutes before turning it back on there was no noise.

As far as everything else goes I have a printer and a wired connection run through a linksys router. And my PC is hooked up to a pretty solid surge protector and as far as I know there has not been any power surges in my house lately. The area I live in has pretty stable power coming in and we don't experience many surges or outages.

Got me on this one. If sounds would be being made, something would have to draw power from somewhere for something?

Yeah, I am kind of stumped on this one myself... Which is why I asked here. I spent at least 30 minutes searching Google and found nothing. Anything I found about PC's making sounds whileturned off came up as a PSU going bad being the culprit.

That could narrow it down as that is actually connected to power, maybe something in that

I'll also throw out it may be a nontechnical issue, such as an insect or mouse inside the case. I've found all kinds of crap inside machines people bring in for repair. Start turning off the surge protector whenever the machine is shutdown and see if the noise happens when the machine is completely unpowered.

It might be trying to do a wake on LAN or any number of things that might cause the CPU to wake up.