Thanks everybody for response and sorry for the delayed answer. I thought the thread was dead do to the absence of mail confirmation.
Anyhow the PROBLEM IS SOLVED.
I used my stethoscope and pinpointed the problem.
Clearly from the PSU (a shitty one that was included with the case, a "no name" brand).
The reason I though the origin was "in the down right corner" was because of sound circulation. Quite difficult to know the origin by ear. With the stethoscope it went like a smack. Quite funny sound too and very discernable. A friend recommended stethoscope and the idea made me laugh. He was serious and said that mechanics use it for car malfunction troubleshooting?
Anyhow, thanks everyone.
A new PSU is ordered. Im going for Corsair TX 850W PSU. Pros, cons? Whats your opinion.
Out of curiosity I also listened to the other components.
CPU: Almost soundless. 120mm fan with big cooler (scythe mugen) so no surprise there. In desktop mode the fan is turned of (steady 50 C). During gameplay maxed out RPM, but doesnt make a difference because of game sound.
SSD: Completely silent (also expected)
GPU: a bit noisy (not audible with ear). But its fan related (stock cooler with small fan). A upgrade to a nice arctic cooler for the gtx 460 is expected.