i dont know exactly what you mean but im taking it you hear more exhaust then engine. you might have a leak in this case and id check for it by starting the car up and reaching underneath feeling around the exhaust of rany "windage" from the exhaust. if you hear the exhaust coming from the engine compartment, check your header gaskets as they may be leaking. i was told by an engine builder as an extra added precaution if you are using the cardboard gaskets, to use a gasket sealer. permatex makes it and is available at your local parts store. it has a copper base to accept the heat and helps seal them. otherwise if you can get ahold of a pair of soft copper haskets, they might help you and are a re-usable solution if you decide to switch your headers at another time