Morning Mark
Is the tapping noise heard when the car is stationary? Or only when moving ? Does it increase become faster with speed ?
If not heard when the car is stationary (engine running) and revved then it probably isn't engine have the belts and pulleys been checked ?
If it is only heard when moving then it could be something stuck in a tyre ? an issue with the drive train, gearbox Prop shaft rear axel. It could be a suspension bush or something loose and rattling (heat shield) is it always on the same piece of road
More questions than answers I am afraid, but you would feel pretty dumb if you went to a Dealer (or any Garage come to that) and it is a stone in a tyre !!
Dave