Not sure if my problem us the same as yours, but I searched and found this post of yours. Mines is basically a rumble noise/vibration when letting of the gas and letting the car coast from 40mph to 30 mph. When I go in neutral it's not a bad.
It is hard to describe the rumble, but it hits more my ears than like a physical vibration. Feels like engine braking or something but it's not doing that.