Knocking sound? Any ideas? I have 4.3L Merc w/ OMC I/O

[mtcaves]

Hello, Motor was replaced last year with a new one from an autorized marine shop. I have a 4.3 Merc I/O with the Cobra OMC outdrive. Took it out this weekend and I have a knock at idle but give it gas (higher RPMs) and it goes away. Hard to hear where it is coming from. Sounds to be at possibly between the Motor and OMC outdrive. Any ideas. I got into a shop today but will have to wait til next week for the verdict.

Thank you in advance,
The Rev. AC

 

[gadget73]

Re: Knocking sound? Any ideas? I have 4.3L Merc w/ OMC I/O

First guess is the exhaust flappers in the Y pipe. If thats what it is (I'm not an OMC guy, so I'm not 100% sure they have these) its not a problem. If the old motor overheated, and OMC does use these, probably the rubber cooked off the flapper, and you're hearing it bang against the Y pipe. Thats what usually happens on a Merc setup anyhow. If its not that, could be the U joints or maybe a gimbal bearing.