To have a chain guard on your rear stay? chain slap is common source of noise.
If its in fact the chain, put a chain guard on the rear stay and check that the chain isnt too long, sometimes the mech can also be broken, allowing it flop around.
Mine does it... I've taken apart linkage components and found a sloppy shock bushing, but I replaced it and it still knocks. It doesn't do it if I'm pedaling hard through the rough sections so I think it's the chain tensioning the freewheel. It doesn't sound like chain slapping against the stay to me. Annoying on an otherwise quiet bike.
I agree Elocma it’s frustrating as hell but I not think it’s the shock, mich be he chain!! Might give the rear tensioner a go and see if that helps!! Mind need to make sure there is a Pugh slack in the chain first!! Emailed intense, just waiting on a response