Weird "flickering" noise on hard drive side

I noticed that sometimes I can hear this quiet "twitching" or "flickering" noise (sorry, I don't know really how to call it) on the side where the hard drive is. Sounds kind of like the noise that old CD/DVD readers made when trying to read scratched disc, but it's very subtle, quiet...

Is that a problem with HD or could it be it's normal activity that I just noticed for the first time because I'm very tired? :D

PS.

Nothing happened to my Mac - he sits on the same spot every day, I don't even move him around much, not to mention bumping into something or dropping... Although, I replaced (upgraded) RAM recently, cleaning his "intestines" a bit with compressed air on the go.