I had a similar prob w/ my 235. I took the engine out before I found out the cause. My hunch (after I removed the engine) was the heat riser in the manifold. My noise sounded like something (bolt or other metallic object) rattling, like something was loose. Never did find it though. Clamp a pair of vise grips on the heat riser rod and see if the sound changes or goes away. Mine drove me nuts too :mad:
