I have this mechanical rattling noise from the rear of the engine, transom assembly area. this is a big block with a bravo one X application. To rule out the drive and u joints , and transom bearing I ran it on the hose without the drive and still have the same noise. It seems to go away over 1500 rpm. I used a stethoscope and the noise is not from anywhre on the engine, checked block, pan, valvecovers timing chain , steering pump, watr pump, bellhousing etc, all sound normal. Strange thing is the rattling is heard on the inner transom plate and steering arm, I see this as strange because with no drive there is no connection to it from the engine. Is it possible that the coupler is bad or loose and rattling at idle? :Maybe resonating noise to the transom? Experts help please before I start tearing it apart more. Thx, Chris

Does it have captains call/ silent choice?

Maybe exhaust flapper.

Silent choice?

no silent choice, I even put the scope on the rear of mufflers and can hear low flapper noise but that is a consistent noise not the sort of random mechanical rattling from the rear of engine or interior transom area

Maybe the inspection cover for the flywheel ? Something fall into the flywheel housing?

Do you have a y pipe ?

What mufflers ? Corsa by any chance ?

I had the same issue, it was the stainless exhaust flapper, one was missing so I removed the other.

Thanks guys,
there is no flywheel cover but there could be something in behind there, I fished a magnet down behind the flywheel and did not catch anything.
There is no Y pipe and I think the mufflers are HPI, I can stop the external rubber flappers with screwdrivers and still heve the rattling noise. there are no internal flappers in the cmi tails ahead of the mufflers. I may just need to pull the engine but would like to know if anyone has seen this, very strange.

Put stethoscope on fuel rail while running.