Check the cheap, easy and obvious first like the condition of the chain and sprockets and the chain's tension. Then have a look for any play in the drive line whilst your at it. Worn sprockets, particularly the gearbox one, tend to make strange rattles under coasting or engine braking as the wear pattern they develop matches the forces and how everything beds in under power - the gearbox sprocket is driving, rather than being driven, if that makes sense.

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