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I had a similar thing recently and it was because I wasn't running the washer that sits on the freehub body behind the cassette. Is there any play in your cassette at all? It is a 0.5mm washer that you need ... but not on all cassettes? I didn't need one on my shimano road cassette, moved to a shimano mtb cassette and needed a washer on that weirdly. Was tricky to find in the end. Presumably they come with the cassette when you buy new but I bought it off here so didn't I have one.
Cassette lockring not tight or missing a spacer behind the cassette? Knackered freehub?