Mavic crossmax freehub doing strange noise at high speed coasting

There's a few different Crossmax/Crossride/Crosswhatever iterations for how you get to the freehub guts to service it. For the one you have, I think your probably just pull off the endcaps, or take them off with a 5mm wrench on each end if there are such wrench flats there. You don't have to take the cassette off; I always do anyway, but it's fine not to if you don't have the tools.

When you take it apart, your chief concern is the pawls or springs flying somewhere and getting lost. It's good to do this operation suspended over a tub or similar so they have nowhere to fly to. The symptoms you're having mean there's some possibility one of the sets is out of place or otherwise unhappy.

Clean everything, and then lubricate it. Mavic freehubs are pretty insensitive to what you use, but typically go with a medium to heavy weight oil. Mavic neutral support mechanics are known to favor Shimano mineral oil. Make sure to keep track of the washer you'll find in there.