Wheels or freehub seem the most likely culprit. Go for the easy stuff first like skewers, freehub if it's an easily serviceable one.
If it happens when freewheeling it's not going to be the RD (unless it's slapping and sqeaking)
Good point on the derailleur.
No idea how to service a freehub, it's a Deore one so presumably plenty of guides online
Eurgh I hate diagnosing unwanted noises
@hippy started
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Wheels or freehub seem the most likely culprit. Go for the easy stuff first like skewers, freehub if it's an easily serviceable one.
If it happens when freewheeling it's not going to be the RD (unless it's slapping and sqeaking)