One also has to remember that all fans "twist" the air... in other words, the air isn't being pushed straight into or sucked out of the radiator. The air rotates like a tornado and that causes a lot of the noise when a fan is too close to the fins.
Another thing that happens is excess air that can't be pushed through or sucked out of the radiator bleeds past the fan blades causing little turbulences and vortices because of pressure differential.
All these things add up to flow degradation and noise. That is a major reason shrouds/plenums help somewhat... the air has a chance to stop spinning and become less turbulent and more linear in flow.
Remember, fans (bladed types) are not positive displacement pumps of any sort... pressure differential between the intake/exhaust sides will always affect noise and efficiency.
Edited by Tex1954 - 7/24/11 at 6:09am