It may be a fan out of balance. Or poor bearings. You don't need to go to a water cooling solution just to fix a vibrating fan. If the fan isn't replaceable, simply replace the whole cooler with a different one.
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.
They are matched fans- there is no marked direction for them to be facing stangly- I'm trying to RMA them, but I doubt they would let me keep this until a new ine is sent with a deposit or something - I don't really want to have wait two weeks for a CPU fan- but I may have to unfortunaly