This is most likely caused by either ‘chain slap’ or ‘chain suck’
‘Chain slap’ is when the chain has bounced up and down on rough terrain and made contact with the chain stay, which may damage paintwork. If using Shimano, ensure your ‘clutch’ is activated on the mech as this makes a massive difference to controlling the chain. We recommend ensuring the high-risk area along the top of the chain stay is covered by the frame protection supplied with the bike, however riders riding rough terrain on Enduro or DH trails may require additional protection. Some of our pro riders looking for a silent bike use a widely available stick on rubber product called 3M Mastic tape. We also recommend keeping the chain clean and using only light or dry lube when used with a narrow-wide chainring to ensure the drivetrain is working efficiently. You could also be seeing the effects of chain suck –
Please refer to the FAQ on chain suck for more detail on the causes and how to avoid chain suck.