Clattering On Over Run

I seem to be having the same issue as CRNeo regarding noise - mechanical clatter on over run

it's driving me up the wall, and i've been throwing money at it since its turned up. so far i've changed:

flywheel - FM lightweight
Clutch - FM stage 1
Gearbox - 60k mile mk2.5 - realigned to 65mm ppf to frame rail spec
N/S driveshaft - found 1-2mm movement in a cv - all 4 cv's regreased
diff - checked for lash + pinion movement, 2-3 oil changes with different oils to see if it altered the noise

starting to think it could be the SS mongoose exhaust - baffles falling apart?

someone did mention the heatshield - one under the boot, is there a mid one? can't remember

haven't checked to see if there's a manifold to engine/box bracket but could be worth checking

any other ideas anyone!?