I noticed that my main home system was making some weird noises. I pulled the covers off, replaced the CPU fan, and blew out all the other fans with canned air. I put all the covers back on and . . . the noise was still there.

Pulling the covers off again, I did some troubleshooting, momentarily stopping each fan. The noise was not coming from any of them!

Then I noticed my video card, an ATI Radeon 8500. Most modern video cards now have a small cooling fan hard-wired to the board, mounted on the underside, where it is not easily visible. Sure enough, this fan had a heavy buildup of dust, and was the source of the noise. A few more shots of canned air and the mysterious noise was gone.