BMW Z3 knocks and feels 'strained' but only when engine temp gets up. I can feel the vibrations of the knocking through the footwell. Not sure where it comes from under the car and have tried to pinpoint without success. It's constant, not just acceleration / braking / cornering etc. Mechanic said rear heat shield was loose? and that he pushed it back up? but when back up to temp it starts again. Could it be heat shield problem whatever that is? Or something else? Strange that its only when temperature gets up to normal, very short trips for example don't get the knocking. Can anyone please help?