Lately I have been having this almost groaning noise when I let off the gas/shift gears. It happens every time, whether it is shifting gears or just sitting still and revving. Seems to be coming from under the car at the back of the engine and maybe more on the passenger side. Exhaust leak is all I can think of, but I thought it would sound a lot different than that. Tomorrow I will get a vid of it happening if I can, but in the meantime any thoughts on this?? In all my years of owning cars I have never heard anything like it.