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problem gears
Just some of them? Could be sprockets moving on the freehub rotor. Make sure the lock ring is correctly tightened, it's one of the threads which often gets too little gronk from people who don't use a torque wrench. The stamped individually mounted ones (5,6,7,8 on the pic, may be different on your cassette) are the suspect ones as they have the biggest radial clearance, so if it's those then setting the lock ring to the correct torque is worth trying if only to eliminate them from your enquiries
Cheers - they did do that, but although I haven't had a chance to properly ride it to confirm, very brief ride down my road cranking the problem gears suggested it's made little to no difference... partly why I'm a bit dubious about the actual source! I will investigate a bit further over the weekend and see what I can establish for myself...