How many other fan's do have in there, do the fan's speed up under Cpu load, then slow down when the
Cpu is Idle? I had a similar problem with a Amd system(with the HLT command disabled). After doing some research on proper case air flow, positive & negative pressure's I agree with AntiHeiss about turbulence and pressure, I unplugged a couple of fan's the problem went away, Still concerned it may be a PSU issue, I took out the extra fans, I must have had a good balance of air flow theTemp only went up 1 degree. I ran a molex splitter out the back of the case buttoned everything back up no problem's, after connecting the fans to the external connector with the fan's out side away from the case the problem was re-created, even trying fans from mild to wild, my result In-adequate PSU. It's a process of elimination, and finding a good balance for your system's configuration
I may be off base here with a Intel system, but it could be possible that the Bios uses a cpu idling HLT command, which can cause fans to taxi up or down depending on the Cpu load.