I was idling at a stop light. When I accelerated, got VERY LOUD exhaust noise from engine compartment. Apparently, the manifold broke off, according to the mechanic. The flex pipe looks ok, and so do the other pieces. There has not been any noticeable shaking, so I don't think it has anything to do with the motor mounts. I didn't run over anything large that might have hit the underside of the block, and the road was pretty smooth. This happened in the blink of an eye. Is this a known issue? What might cause this?