Hi SextanteUK
As others have said, it depends how well they are set up.
I have experience as pilot in a few.
Bell 206 a nice one quite smooth but with two bladed bump
R44 a new one similar to above a very old one quite rough
R22 similar to above but smaller so not as smooth.
SA341 Gazelle a few different ones ,smoother than all above.
As a pax in
MD500e 5 blades felt smooth to me
EC155 various, similar to Gazelle some better than others
Are you talking machine vibrations or turbulence ?
Also in particular The R44 6 cylinder engine vibrates a bit through the
Cab in comparison to a turbine
R