Most likely just a simple cable adjust at the derailleur as many have mentioned. Odds would be the cable has slackened so you would “unscrew” the barrel adjuster counterclockwise. If you have a stand you can mimic tightening the cable by lightly pull on the cable at the downtube run. If the chatter stops, you need to tighten.
Other possible, less likely causes:
-Chain is stretched and worn
-Cog is worn
-The cable or housing is worn or oxidized and not moving freely
Twer me, I’d start with the Park video in the earlier post and see if you can dial it in with a cable adjustment. Good luck!