Go to Apple Store or other place which can check and replace it under replacement program.
It’s a common rattle, which cannot be fixed by you.
See Rattlegate thread in Macrumors.
Personally I had this issue with Noise Cancelling enabled - it was replaced in like 3 days after I left them for checks in Poland (we don’t have official Apple Store, only Retailers)
And official:
Need to repair your AirPods? See your service options, their costs by coverage type, and how long they take.
support.apple.com