Could well be normal. have you checked the rpm and then used a different gear at the same revs. Does the same vibrations occur. If so its the engine, if it only happens at the road speed in a different gear then it is the drive train or tyres etc.
In my personal opinion it is probably normal, the fix is to go faster (or slower so it dosn't occur)
In much the same way that ear plugs make rattly engines go quiet!
beeman
