Weird clicks strike back. Still PSU relay?

[Kajeto]

Sooo. I have to apologize upfront for putting your knowledge to use like, everyday, but you guys helped me already by so much that now I cannot resist.

Few days ago I made a topic regarding weird click/tick sound coming from the case. Still, what happens is that, from time to time, mostly under load, I can hear a very particular click sound coming from inside the case. It never persist, it stops after single "click like" sound. My mind has been at ease since Archangel1994 stated that the very sound is made by psus relay. But... From what I hear now, the click doesn't even come from the PSU part of the case. What is most awkward to me, it almost seems like its the case is making those clicks. Like if pressure was accumulating in a certain point, and after reaching its peak, it just dissipates in a process hearable by the disturbing click itself. So... If there is like, any chance, that something might be screwed too tightly, or anything like that? It sounds like if there was, for instance: a match or something similar hitting the side of the case from time to time. Im not by any means disturbed or worried about this sound, as I hear it since I first launched this rig, but... I just wanna know what might it be.
 

[Imabigmac]

So if it doesn't sound like it's coming from your PSU, where is it coming from? - what zone of the case? 

[Kajeto]

Front side, I heard it coming from rather mid section, where the hdd/sdd section ends.

[vlkn´]

The sounds you hear, is the HDD, writing and reading Data. Normal.