105 cassette, Bottom two gears are solid, top two are solid. Middle six have fore/aft play which is rattling.

Nothing looks worn. Any ideas? Is this just a feature of mavic freehubs?

missing the thin spacer?

Wot Al says.

Nope. No side to side play.

You need to tighten the lock ring

Nope. No side to side play.

the answer would be Nope, the spacer is fitted. Is it?

Aksiums dont have play as standard, but they do need a thing washer/spacer with 10spd, and a loose lockring will make a cassette tinkle or rattle.

IIRC, and I might not, the largest sprockets are in a block of three so you shouldn’t get only two that are loose, the others are separate.
Take the sprockets off, clean up and put it back together.
Make sure the lockring matches (11T and 12T available) and torque it back up HTH

Sorry should read “only two that are tight” doh!

Had the same my brand new planet x carbon cx bike .fitted a spare spacer that i had . sorted .

Spacer FTW here too…

is it a 1mm or the mavic 1.75mm spacer thats specific to there hubs

Assuming the cassette is one piece and does not require a nip up. Gear indexing not properly set on the cassette causing this? Just guessing…

Extra spacer sorted it.