Yes those. Pretty sure they need a washer/ shim above ,below or both. Depends on drive alignment. Dont order these until you have someone knowledgeable look at it.
https://nuwavemarine.com/oem-mercrui...RoCML4QAvD_BwE

If the boat was run much like that it could wipe out the coupler or gimbal bearing or both. That could cause the noise. Unless you have access to a drive stand/lift, the boat needs to go to shop to have the drive removed and inspected. as well as the coupler. inspected. If the drive is off replace the gimbal bearing so you dont have to take the drive off again next summer.

Chris

I got the information it should look like this.
There is a spacer (e) between the bolt (c) and the rear engine mount (a).
So I think its all fine, it only looks like there is a gap :)

https://cimg0.ibsrv.net/gimg/www.off...71041fc5e4.png

Yes but is it spaced and drive aligned correctly. I bet if the drive is pulled the noise will be gone.

Chris

You said
So it makes the same sound with the drive off, but the sound does appear that it is coming from the rear of the motor.

Can you remove the drive and post another vid with the motor running?

Yes, the knocking sound is there with the drive removed.

Here is a video from a couple of months ago when I had the same problem with the knocking sound and the rpm problem. Here is the drive removed from the engine.

edit: you can hear it in the beginning of the video

I'm thinking either the coupler is loose (bolts may have come off or loose) or the flywheel is loose

Is there internal baffles or flappers with that exhaust? Is the exhaust all tight all the way to the tips and no leaks?

Yes that was my thoughts too.
But after I drove the boat for a while yesterday the knocking sound disappeared. Really strange.. I could here it the first hour of straight driving in like 5 knots but then it suddenly disappeared..


Yes there is internat flappers at the endpipes of the transom. As you can see in this picture.
The exhaust is all tight. I made new tailpipes because the old CMI tailpipes was leaking.
Now the new ones are leaking too.. but I dissconected the water from the leaking point and now the rpm is much better. So its like dry tailpipes now.. (Im going to take them off and get it welded again so it doesn't leaks).

But it feels like theres still something that isnt right whit the engine rpm..
When I drive the boat in like 5 knots it behaves really strange. Its going up to like 1000 rpm from 700-800 rpm sometimes, and then back up to 1000 rpm and then drops again spontaneously..

https://cimg3.ibsrv.net/gimg/www.off...87a9d94c1.jpeg

Have you replaced the IAC as BUP wrote in one of his posts? Common problem with these engines.
Check your sparkplugs and wires to make sure it fires on all cylinders.

Mine has a rattling noise at low rpm´s that I know is caused by my outside flappers but not as loud as yours.
Is the noise related to engine temp as it disappeared after a while when running under load?

Old school cheap stethoscope such as long screwdriver, aluminum boat pole with rubber ends removed, etc. and touch to various places on engine? other end to your ear?