I just replaced my impeller on my 200 hp mercury outboard EFI, so I may be just be listening too closely. When my boat is in gear at very low speeds, basically in gear and at idle, I hear an intermittent rumbling/slow vibration coming from the motor. I does not sound like the motor itself, because I cannot hear the motor missing at all. Im not sure what it is. Any ideas??????? As soon as I get on the throttle, it is unnoticeable.

Could be something as simple as a hood latch out of adjustment.

I had something like that on a new boat with a 200 Merc. It was a washer vibrating on the steering linkage, LOL. Drove me nuts for several weeks. About a 1'16 turn of the nut, maybe just a little less, and know more rattle.

I would check and see if the prop is tight.

Check the nut on the flywheel. Can cause that type of noise.

Just to be on the safe side I'd check the prop and the flywheel for sure. If the prop is loose you could loose it and if the flywheel nut is loose that could damage the flywheel and the crankshaft. Either one of those issues could get real expensive........

Thanks for the advice. I stopped by the storage on the way home from work this morning and yanked on the prop. Feels like there is about a 8th of an inch of play. Im gonna pull the prop on my day off this week, replace any worn washers and re-torque the prop nut. I will check the fly wheel nut as well. Thanks!