Funny enough, my old Merc 2 stroke used to rattle/vibrate/buzz bad enough to make things rattle in the side pockets, but, only at low speed, when crabbing I have quite a distance at 4 knot speed limit, which is right at the vibration speed, so I had to speed (only a bit) or go idle slow, it was perfect above or below that certain RPM/speed, never heard of it on a 4 stroke though.