3GM Transmission Rattle/Vibration (KM3V)

[theway]

Can anyone help Identify this noise?

https://www.youtube.com/watch?v=8LCj9WYDEK0

I clearly noticed the sound coming from the transmission when sailing around Cape of Good Hope when I turned off the engine but left the drive in neutral. I couldn't be sure but the rolling of the boat seemed to be a possible cause (or directly related) The shaft would spin at different speeds as I sailed down waves, but speed doesn't seem to change the sound much, more so the angle of the boat. Later when motoring on flat water there was no sound.

(Background)
The sound started when crossing from Maldives to Seychelles when motoring but I couldn't Identify where it was coming from, because I was motoring. I figured it was related to engine vibration somewhere. It happen very rarely so I didn't pay much attention.

Then, from Seychelles to South Africa, I motored a lot and never heard the sound a single time.

[tkeithlu]

I think, underline think, I am hearing two gears that are moving in and out of full engagement, burring against each other and then going back into alignment. My guess is that the different forces on the prop as you are going down waves and rolling are triggering it. Everything is staying together under the more constant thrust of motoring on calm water. In short, a bad bearing.

But do eliminate sources outside the transmission before you tear it down. Be sure that the sound is not coming from the shaft, from for example a bad Cutless bearing allowing the prop to just knick something.

It's frustrating to try to ID the sound when I can't get any directionality from the video, and I can't put my hand on the shaft to feel the vibrations. Hopefully someone else has some ideas as well.

Oh. If it is coming from within the transmission, you should be able to feel the tapping by putting your hand on the transmission. I'm hearing impaired, so I track down noises with my hands.

[Wotname]

My guess...

It sort of sounds like a cone clutch slightly engaging with the torque being applied from the wrong end of the clutch (i.e. the prop). This might be possible if neutral point of the transmission is not correctly adjusted or has worn.

Tkeithlu has made some good suggestions!

[theway]

Thanks for the suggestions.
Is there a way I can simulate the situation to find out the exact cause? Or tests I can do?

The only thing I can think of now is trying to spin the shaft with my hand, but when I do I’m not getting any sound or vibration. Probably because there are no other forces on the shaft and/or I’m not spinning it fast enough. How do I simulate a situation to reproduce sound. I guess I’m gonna try to do my haul out sooner than later. The water here is too cold and murky to do much investigation/work in the water.

[tkeithlu]

It's imperfect, but start by looking for play in the system between the prop and the transmission. Is the prop snug? is the Cutless bearing OK? Then disconnect the shaft at the transmission and see if there is any play in the shaft or in the output flange. Whatever is causing this is loose. At least eliminate shaft/prop sources before you tear apart the tranny.