Thanks for the responses!
It’s a hard tail so no worn Sus bits.
No play in the wheel bearings at all.ajd they’re not fitted too tight.
New freehub does exactly the same thing so I’ve ruled that out.
I’ll definitely measure the chain later using the Shimano method though it’s the one that came with the bike so I’d assume it’s correct.
I’ve stripped cleaned and regressed the whole thing so it’s totally clean including the chain, cassette, and i don’t think it’s cassette inertia as I wouldn’t expect that.
I don’t have another 12 speed boost wheel to try or get would be doing that next to rule the wheel out.