Experience some low "boom" type noise at low speeds, usually when slowing down or at idle. Thought it was a bass sound from another vehicle, but no. Vehicle is 22 GT vert, stock except some sound deadening material on bare metal parts. Anybody experience similar issues?

At lower speeds, usually slowing down and at idle, hearing low frequency noise much like coming from a sub-woofer. Radio is off. Car is 22 GT vert stock. Any ideas?