help on rattling noise from rear of engine
 

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.

Have you pulled the drive off and checked to see if the phantom noise disappeared?

May be a "Way out there" item to check (and easier to check than pulling engine) as most times it sounds like a "grinding" or "howl" that is transmitted through the transom assembly.... but, if the big Tapered roller bearing on the top of the drive carrying the input shaft behind the yoke is on its way out/ or merely several rollers have flat spotted on the bearing, it will be hard to detect and doesn't transmit from outside the boat (unless really bad) - just behind the engine in the compartment.

.. and yes a bad bearing can/ does stop making noise at a certain rpm as the harmonics level out.

Sometimes the exhaust pulses echoing off the transom can cause a rattling/clunking sound while idling...

loose rocker arm??????

Rubber flaopers won’t make the noise but loose metal in the mufflers sure will.

I’ve seen multiple Corsa’s do this, thus why I asked. Doesn’t mean HPI’s are immune.

both mufflers sound the same with the scope so I think I can rule them out. per above on fuel rail scope I have a holley carb so no fuel rail here.
I will try to see if the coupler is loose somehow before I bull the thing. thx

I have the same noise in my boat. Pulled the 330, put a 420 in it with new exhaust and tips, reused coupler and bellhousing, still the same noise. I would say it has to be one of those things. Nothing visually wrong with them upon reinstall 🤷‍♂️

There are typically metal valves that open in Formula's tips, inside the flapper. Take off the exhaust tube ad look at the tips fro the inside of the engine compartment. They rattle.

put on a new coupler and new coupler bolts, the ones with the shoulder for tight fit ent to the coupler holes. someone had just used regular grade 8 bolts. Noise fixed, all good now.

Just because the drive is removed, a rattling coupler will still rattle.

So what was the noise? Defect in coupler, wrong bolts so coupler wasn’t actually tight to flywheel, or wasn’t centered because shoulder bolts weren’t used? Weird that it made noise even with drive out and no spline shaft in coupler. Kudos to you for finding that.

prob loose bolts for the coupler. Seen and heard and fixed that before.

Volvo penta had a SB about this exact issue.