Recently purchased a used boat which has a very unusual sound in the engine compartment. The sound is an occasional thud. Sound is random a not tied to engine rotation. Its deep and can be a single thud or 2 thuds. Does not sound like any typical internal engine issues. Sounds like it originates back by the bell housing. I have pulled the drive and started and the issue is still present. Just looking for some pointers as it appears I will be pulling the engine to dig deeper.