Well, I have had a mysterious noise coining from the engine compartment when I cold start the car and run it in reverse for a minute or so. Goes away after a minute, or when I shift into DRIVE. Had a diagnosis, engine mounts looked OK but torque strut was questionable. Had that replaced. Alas, I still have the noise in reverse from cold start. Really appreciate all the troubleshooting that you folks did, but I guess I'm stuck with a mystery noise.