Hi Guys, I have a 1991 Hunter 30T with a Yanmar 2GM20F engine. The engine has been running great and all the fuild levela are ok. When heading into the marina running approx 2500 RPMs I heard a loud rumbling sound for a few seconds(sounds like a car had blown off a muffer). I brought the RPMs down and continued to head in. A few mintues later it did it again for a shorter period. I made it back in and checked over the engine and everything looked ok and water was flowing out the exhaust. Reading some the articles there was talk about if cooling water was not flowing then the engine would sound different. Does any one have any ideas? Thanks Jim