Thanks for the suggestion, I've installed a cheap rubber coupler and it runs much better than the originsl, but I think I might try the hyco one next

They run very smoothly better than the rubber ones

You could try out one of these prop shaft fittings. If you are slightly miss aligined you could sometimes get away with these. It's a Hyco fitting, they are on ebay and quite cheap.

I will check that and see if it helps, thanks for the comment.

Thanks for the helpful comments, I will post an update when I've done some more tests and solved the problem.

Misaligned or bent prop shaft and lack of support for the prop tube.
I suggest you take the motor and joiner out and check that the prop shaft is running true. If it isn't then you will need to correct. As you are running a high speed brushless the whole drive train need to be absolutely in line or you run the risk of major damage due to the forces involved.
Hopefully the shaft will be running true so you can then attach the joiner to the prop shaft and check that is also running true. Then check that the joiner is running true when attached to the motor.
Assuming all is OK the problem is with the motor mount. You will have to enlarge the mounting holes on the former to allow you to adjust the motor and may need to add some shims between the former and motor to align correctly. Might take you some time but will make your model so much better.
The rubber coupling will be beneficial and should help reduce engine noise. It will not however cure any misalignment problem.
Support of the propshaft near to the coupling will also help.
Looks to be a nice model and well built.👍 Hope you manage to cure the vibration.
Dave976

Hello
Miss alignment sounds like the problem.
You may have to change the motor mount and make up a new one.

I replaced the whole drive line with a 4mm one and a 28mm brushless motor.
Canabus

as a guess do you have enough grease in the tube for the propshaft
to spin without binding?