Grumble/rumble under deceleration?

[kilr4d]

Got a rumble under deceleration that's quite noticeable under 10 - 15mph.

It's very weird....doesn't quite sound like a wheel bearing. If I had to describe it, it almost sounds like gear not meshing or a faint ratcheting sound. Imagine a cross between the sound of heavy lug tires and the sound of a parking pawl when the vehicle is still moving?????

Any ideas????? Driving me insane.