Rattling noise over rough ground

Hi all, 

I’m experiencing an annoyingly loud clattering/rattling noise from the rear of the bike when freewheeling over rough uneven roads, this noise disappears when rotating the pedals..the chain tension appears to calm it down? The cassette lock ring is secure, checked all spokes, even taped up the long valve stem but to no avail! I used my g/f’s wheels which are exactly the same and no noise at all! 

The wheels are Mavic Cosmic Pro Carbon 2016 being used with an 11 speed Ultegra cassette. both parts are approx 3 months old.
I’ve read elsewhere that some have used a shim/spacer with Mavic wheels behind the cassette even though it’s 11 speed and not needed? 

If anybody can offer me some advice or has knowledge of this issue I’d be grateful. 

Thank you!