Oh my goodness this is irritating beyond belief. It's like that cricket that's outside your window on a hot summer night that you can't find and kill because you're trying to sleep.
I tap my case lightly with my fingers and it goes away for a few minutes and then comes right back. I took the side panel off and put my finger on the mounting area for hard drives and felt it vibrating real hard, when i put my finger on it the sound stopped.
What can I do about this? My buddy told me to put that foam stuff into my pc case, this stuff:
Random vibrations can be really annoying. I normally use little bits of Blu-Tack around the contact points of whatever it is. Another option is to cut little strips of thin felt an put that along the mounts. If that doesn't work, just tape a weight to the mount to simulate your finger. I know this sounds dodgy but it is by far the cheapest option. If you find something that works, then you get neaten the solution from there.