My 716K has a rattle and I'm struggling to find the cause so any suggestions welcome

I can't hear it inside through the floor/engine cover, only through the window especially when alongside something reflective like a concrete wall.
It's only when driving, speeds up with road speed, can't be heard below about 30mph.
It sounds as if the tappets need adjusting.
It's not gear dependant, just speed.
But is instantly turn on and offable with the throttle. Throttle on even a little it starts, let go it stops instantly whatever the road speed is.
So maybe the twist of the engine under power brings something closer than under no load, but what?
I hoped it was as simple as the chain on the battery cover pin but no

Any ideas?
Cheers
Jason