I'm new to the site. I aquired a 1976 mercruiser 165. Thanks to you guy's on this site, I was able to get the boat running. I put in a new water pump, impeller, fluid, ect. With the muffs it appears to run fine. I took it to the lake to try it out. Left the dock, past the 5 mile buoys, gave it the gas, no problems. Went about 300 yards and heard like a rumbling noise. Shut her down, put in neutrall. Still had the noise. It appeared it was coming from the outdrive, but could not pinpoint. Went back to the dock and took her home. Put the muffs on, and there's no noise. What do I do next? Any help would be appriciated. Thanks, John