Too many possibilities without knowing a bit more. Describe the sound, does it happen all the time, only while coasting, or only while pedaling? Is the volume related to how hard you're pedaling, ie louder when hill climbing? Is it in all gears or only certain combinations.
Don't take this wrong, but we're not psychic (except maybe the Operator) so we need good information from you to be able to provide any meaningful answers.
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