Help - Engine knock in a 2000 model Wellcraft, 186SS,- 4.3 Litre 190 HP Mercruiser

[seizethecarp]

As I mentioned it's a 2000 model Wellcraft, 186SS, with a 4.3 Litre 190 HP Mercruiser engine. It has 172 hours on it and developed a knock last time out on the lake. A local mechanic at the slip disconnected the stern drive from the engine to isolate the noise and was his opinion without going into it that it could be the balance shaft. any ideas or suggestions on what this could be and how much it would cost to fix it?

thanks guys