Hey Michelle, what you're hearing sounds to me like a harmonic vibration occurring. Unfortunately, it can be just about anything from a transmission problem to a wheel bearing, alternator or water pump bearing, to something under the car catching in the wind.
I'm betting that unless you were with the mechanics that said they saw or heard nothing, they heard it but decided to pawn it off on someone else because of how intermittent it is and how hard these things are to track down.
Unfortunately, the only way I know of to help you figure it out will require you to basically empty your wallet until it's found and fixed.
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Saturday, August 29th, 2015 AT 3:37 PM