ok...my other drive doesn;t do it and 15 high performance boats at my marina don't do it

I Appreciate it.. we are a bit far./ I am near Westchester on the Hudson River

So we are pretty close then....I'm just north of Haverstraw Marina

Put your hand on top of the drive, you will feel a vibration from the shift fork cam bouncing up and down as the bump rings spin around and the space between the gears, moves up and down. Now shift it into gear. It should quiet down, as the linkage is holding towards the gear the cone is engaged with. Some make more noise than others, depends on gear stack, etc.

Hope that helps.
Dick

Update......I was out on 2 runs only this season so far and now the noise got much louder....No unusual vibrations felt...
check this out

I am going to suggest getting two 10' lengths of plastic downspout corrugated pipe (solid kind, not holes), and tape it up to your exhaust to help isolate the exhast/motor noise away from the drive when you are troubleshooting and filming.

I don't doubt you that you hear something in the drive. I can hear something in the video, but I can't discern any details about it.

buy a real doctors stethoscope. I have been using one for 15 years now. It is used for multi testing marine engines and noise testing , fuel pumps, fuel injectors, bearings in pulley's and of course outdrives, .

Wonder if its U joints. - When ever I pull all that assembly apart for changing U joints , I mark the shaft - yoke and input shaft . -- then put it all back together by the marks I made. Line to line to line. This keeps the balance of the assembly like how it was.

Pull the drive off --- stick a quality alignment tool in the coupler -- start the engine - How much of a perfect circle is the alignment tool making when the engine is running ? If not, then the coupler is warped or loose or coming apart. That could be the issue.

As mentioned swap the good drive over and then run the engine to see if noise goes away. If so, its a drive issue / or U joint issue. If not then no drive issue or no U joint issue.

I appreciate your input, good points...as for the coupler, Wouldn't there be a noise behind the engine if the coupler was the culprit.. I can stand in my engine compartment with engine running and i can get behind the engine. All i heard was the engine. but the knocking noise you can only hear while standing behind the boat and you can here it over the exhaust noise.. As for the u joints..I replaced the complete input shaft assembly.This comes with u joints, front shaft, rear shafr coupler, everything except the gears and bearings..They checked out fine...The only thing i notice now is the noise is louder. So i will be bring the boat to the marina and let my mechanic listen to it...Its hard to pin point it in the video..but listening closely there is a rattle besides the cam sound that is not even with the cam.

I had a drive do this once and it was the gimble bearing not tapped in far enough. was the strangest thing.