I noticed yesterday when i took and changed around my air flow, i noticed a drastic change in the sound. I used to have like 3 going out and 2 going in. The reason behind this is the simple fact of air pushing out on the other fans. lol. but it dropped it down alot, and you can put your fingures along the cracks and feel the air comeing out. Try positioning of your fans, loud fans are good
at least you know they work.