Took both tyres off, drained the water out of the rims, inspected everything, replaced tyres with talc (thumb pressure only needed with talc to dry lube them) then started on the cassette and freehub. Stripped everything, cleaned up are re-lubricated the Mavic freehub with 3 in 1 oil:
This is the very simple design of the Mavic freehub; you can see how it sits on a plain boss lubricated only by a thin film of oil. With insufficient maintenance there is direct contact between freehub and boss - you can see some scoring in the photo so it must have happened at some time with mine. You can also see the spiral ring machined inside the freehub, which is supposed to spread the oil around. There is a rubber seal, which goes in the hub at the bottom of the boss. The design is easy to strip and good in that the drive-side wheel bearing is well outboard giving good stability; that it's small doesn't seem to matter because it stays clean and dry so doesn't wear prematurely. However with poor maintenance the whole freehub setup reaches a point of sloppiness where it oscillates fast on the boss and you get the Mavic howl of death, which means it's time for new wheels. I tried a thicker engine oil but that created too much drag and the cassette was getting pulled around by the wheel when freewheeling, causing the chain to sag so I went back to the recommended 3 in 1 oil.
Like I said, a clever design for its light weight, simplicity and ease of maintenance but not neglect-proof like sealed Shimano freehubs. It needs to be stripped, cleaned and re-lubricated every 6 months. Buy Mavic wheels and you buy into a design philosophy, which some people don't like.