IF the rubber shock absorber's rear wheel are worn then IMHO this could cause the problems your talking about. Combine that with a potentialy worn chain and maybe even sprockets then the vibes are gonna be noticable.

Chain maintenance is often overlooked. Cleaning and lubricating it is high on my list of bike prorities.

My only moan about the R is it has no rev counter,so its hard to identify when the noises/vibes appear.

TBH I have noticed that if Im lazy and dont use the gears in the correct range etc then my R does kinda vibe BUT if Im on it then she's sweet.

FYI there is a 'fix' for the rubbers. Basically an old inner tube cut into sections and placed between the rubbers and the casings. Cheap and cheerfull.