340 vibrates

My 318 does the same thing. Had it out for a drive today and was annoyed by the vibration going down the road. Kicked it into neutral for a minute at 60 mph, vibration and associated noise went away.
It also does this sitting still, in neutral, or park, rev the engine up a little and you get that rhythmic vibration, comes and goes, same thing that I feel when driving down the road.