I have this weird smell emanating in the boat - not all the time though. It happened once when I got the boat but once I had cleaned it top to bottom it was ok. 4yrs on and I woke to find this smell had returned but was making me feel sick enough to pass out and I couldn't locate the source. the engine had been run up but this was middle of the night. I checked the engine bay and the waste tank (source of original smell) and all was clear but when I pulled the under sink cupboard open the smell was really bad enough to be ill again. I didn't find any smell in the engine again and the head / shower areas and even checked the waste tank from the shower. It came as quick as it went after venting the boat 10 mins so am thinking its the head system pipes behind walls or the fridge gas system???? I do have water in places where there was none before ( or cleaned out from the start ) so not sure what or where it came from. Btw filters etc have been changed and full engine serviced risers done etc so nothing mechanical like oil or fuel..... oh that's the other fun things in my bed under the deck has water from someplace getting in...….