Is it a Mavic wheel though? If so it’s almost definitely the Mavic scream of death.
Edit, just read that it is. Starts off occasional and ends up with your freewheel getting stuck and the chain going slack/fucking shit up. You should give it a good service as @farewell said.
I would double check the lock nuts especially if the wheel has been serviced recently, sometimes if their not tight they can tighten up causing the cones to grind against the bearings
Is it a Mavic wheel though? If so it’s almost definitely the Mavic scream of death.
Edit, just read that it is. Starts off occasional and ends up with your freewheel getting stuck and the chain going slack/fucking shit up. You should give it a good service as @farewell said.