Spun coupler or cavitating / spun prop. Does the prop appear to be in good condition? I would pull the prop and inspect for evidence of slippage. If the back of the engine is accessable, check for rubber fragments around the area of the coupler. <br /><br />If nothing is found, pull the outdrive for an inspection of the u-joints and gimbal bearing.<br /><br />DaveOriginally posted by firestorm:<br /> My boat gets up on plane and then starts to make this rumbling noise. The whole boat starts to shake and it loses power.