Hi guys,
I was wondering if any of you guys ever had a knocking noise like the one in the link below?
The engine is a Mercruiser 454 MAG/EFI 385HP. I used a 4' length of garden hose as my stethoscope to track the noise and it seems to be coming from the back of the engine, behind the distributor/transom area. Has anyone ever had this problem with an engine like this?

The smoke you see in the video is fogging oil burning off. This is a video of last seasons first startup.

https://www.youtube.com/watch?v=P2FnlFBM5_A

Do you have internal flappers on the exaust? Stick a broom handle in to wedge them open and see if it goes away, mine make a knocking sound like that at idle.

I had a baffle in a Corsa Muffler tip partially break free and do this too....above boat doesn't have these.

By your description, and great vidieo btw, and the tach not jumping I'll agree w/Mr Maine for now.

This might be a dumb question but is the noise always there, or does it go away? and is it only at Idle or though out the rpm band?

it,s hard to tell but i don,t think the noise is the exhaust flapper.i noticed when cranking the engine before it started that it sounds like one cylinder lacks compression.i would do a compression test and if a cylinder has no or reduced compression pull the valve cover on that side off,you might find a broken spring.

Does not sound like an internal exhaust flapper. Its to rhythmic and sounds like its in the valve train.

i hate to say it, but it sounds like a rod to me...

.,

First thing I heard was the rhythmic sound of one cylinder low on compression like mike said.

Its hard to tell 100 % from the Vid. Just saying the noise sounds fast and rhythmic just to point out the camshaft turns half the speed of the crankshaft. IMO the noise could be below the valve train components - ie: lower end of engine and or like some said compression issue. Very hard to exactly pin point just watching a video - not having ears, eyes and hands on it.

Look in or pull your valve covers - any metallic coloring - check you motor oil for metallic and cut open the oil filter, compression test - exam spark plugs - just saying for the easy stuff first

For noises I really like a quality real doctors stethoscope. That thing has paid me 1000 times over.