Thumping noise going down road, sounds like bump on left rear tire but is in engine, drive train somewhere. Can feel and hear with hand on engine at 1200 and 1900 RPM but cannot hear on engine with stethescope. Can hear on ex. pipes and gutted cats but no change. There with serpentine belt off. Do not know age of timing belt, is there something in timing gear that will cause this? Is there on deceleration also. seems to be getting worse and is worrisome. Any one have any ideas on this please reply. It is not in left rear wheel.