I have a 1987 35.5’ C & C MKIII. It has a 3GM30 Under the hood. love this very resilient engine. It’s not the first one I have owned. My former Hunter 36 had the same engine. Ok so the C & C is new to us. The engine starts and purrs like a jewel. Except for at very low base idle there is a somewhat loud chattering noise heard and felt in the cockpit. The moment you raise the idle even a couple hundred rpm’s the noise disappears. I’m a former automotive technician turned automotive engineer retired. So I’ve always diagnosed and repaired my mechanical toys And still do. At first I thought it was valve lash but now I think not. It’s currently winter in the Great Lakes area where our boat is hibernating. I live 700 miles south. So I can’t get my hands on it until spring launch. And I have not put my stethoscope on it either-yet. So I’m thinking maybe there is some part of the assembly where the transmission connects to the engine that has a worn part or maybe an adjustment to lessen lash. If anyone has experienced this on a 3GM30 I would really appreciate your comments here.