Either, I can switch em and the they stop the vibrations only if one of the two are unplugged- I keep the one on because it is faving the exhaust vents on the top of the case.
Strange. It is like they are creating turbulence between themselves. Are they matched fans, running at the same RPM? I would try swapping them around and see if that makes any difference. Also, remove them from the cooler and verify that the air is actually moving in the direction of the arrows on the fan housing. They may have been miss-wired at the factory.