I found out something awkward today, don't know if it is know among other silent/quiet pc enthusiasts.
my current setup contains:
-Antec Solo with 1200rpm slipstream slim rear fan
-Scythe Katana 3 CPU Cooler
-AC Accelero Twin Turbo (at 7v)
basically what I've found is that normally I can hear a wooshing sound coming from the chasis. I'm not good at describing noise characteristics nor have precise instruments to record the noise but it basically sounds like what you hear with wind blowing past your ears.
Today I felt like making 100% sure that I know which fan is making that noise before I buy new part for my PC.
So I removed the side panel and... the wooshing sound disappears, noise from my pc suddenly became much more tolerable. I put the side panel back in and the wooshing sound returns.
Note that I could hear the wooshing noise before I have swapped out the antec tricool with the slipstream slim, so it doesn't appear to have much to do with the scythe fan.