Out on the bike today when I stopped pedalling the cassette kept turning which in turn bunched up the chain
Know feature of Mavic hubs.
Gooogle for “Mavic Crossride Freehub Squealing” (or Aksium, etc. since the design is the same) and you’ll get hits here and elsewhere.
There should be a washer that goes behind the freehub when you re-fit it, and a rubber seal presses against a yellowish PTFE disc. Don’t use any grease in there, but only a light machine oil, else it’ll drag.
edit: and mine did it on holiday in the Cairngorms. fun having to permanently pedal like a fixed wheel bike thru all the berms and jumps at Laggan. There is a 3rd party replacement freehub seal kit available on ebay, but as my axle also bent a bit I didn’t bother fixing it.