i HAVE 120 MERCRUISER. HOOKED THE HOSE TO IT AND FIRED IT UP RUNS SMOOTH BUT WHEN I REV THE ENGINE I HEAR WHAT I THINK IS A LOUD VIBRATION. SOUNDS LIKE ITS COMING FROM OUTDRIVE. IS THERE A FLAPPER FOR THE EXHAUST OR SOMETHING. DONT WANT TO GET ON THE WATER AND HAVE A PROBLEM. ITS A I/O