sounds like a vibration issue. You need something like a rubber gasket between the fan and case maybe. Something like this (but to your size and in stock):http://www.newegg.com/Product/Product.aspx?Item=N82E16835165003
I have heard that you can also use a very large rubber band, cut into a small piece with a hole in it.
Put your mounting screws into the fan, then into the hole in the rubber band pieces, then mount your fan to your case. It should give you a home made gasket to reduce vibration.
Does the sound change when the fans are flat vs upright?