This is a more detailed answer. I cautiously don't agree with Chris that the OP needs to stop riding the wheel, but it does depend on how the rim is constructed. Basically, the aluminum piece is a sleeve of some sort. The sleeves are structural elements in some but not all rims. I believe it is not a structural element in this rim, which the OP described as a 2008 Mavic Ksyrium Equipe. I shall explain why, but this requires a review of how rims are constructed and joined.
A review on alloy rim construction methods
There are 3 ways to join rims. Astral Cycles is a rim and wheel manufacturer in the US, and they explain:
A: To understand why the joint matters, it is important to know the
process. We roll our extrusion into a helix that will eventually
become two rims. After a rim is rolled, it is cut and then requires
the two faces from the cut to be joined together. There are three main
ways to join a rim: pinned, welding and sleeve.
Pinned: Only the cheapest of rims are pinned. Pinned uses two steel
dowel pins jammed into formed or extruded holes in the rim to hold the
rim together. Pinning only holds the mating rim faces at the two pins
so the brake track and spoke bed of the rim have a lot of flexibility
to move relative to each other. This is why some cheaper rims tick in
the brakes at the seam.
Welding: This came to the forefront to replace pinning on nicer rims
about 30 years ago and was a big improvement. Welding can create a
nice, finished looking joint but it has some tradeoffs. The material
in a rim is heat treated to increase the strength of the material.
Without this heat treatment step, aluminum is not strong enough to be
a bike rim. The flash welding process generates an enormous level of
heat at the joint area creating a “heat affected zone”. This heat
affected zone degrades the heat treatment of the material surrounding
the joint making the material at the joint weaker than the material
everywhere else in the rim. This is not to say it doesn’t work, just
that in our experience this unnecessarily makes the joint the weakest
part of the rim. We have full welding capability in our Oregon
facility, but with all of our experience, we have decided to sleeve
join our rims.
Sleeve: Sleeve joining uses a form fitted sleeve in the exact shape of
the inner cavity of the rim to span the joint. It is about 2.5” long
and weighs typically 4-5g. In our process, this sleeve is a press fit
- meaning it is incredibly tightly fitted and requires a specialized press to insert. We use epoxy as an insulator to minimize the
potential for any noise but the epoxy is not a structural part of the
joint. What we like about this process is that the sleeve effectively
doubles the wall thickness in the immediate area of the joint making
it the strongest part of the rim and it does not affect the heat
treatment of the material. We then machine the brake tracks after
joining so the brake track is completely smooth.
One further point: at least some welded rims are held together by a small sleeve during the welding process. After welding, the sleeve plays no structural role, i.e. it does nothing to hold the rim together. The sleeve also acts as a heat sink during welding. This may reduce the probability that the rim warps, and it may mean that the welding process doesn't affect the rim's heat treatment as much.
So, what went wrong here?
This non-structural sleeve can sometimes come loose. This link from Wheelworks, a New Zealand company, documents this assertion. It names Pacenti and Stans rims. I have personally had this happen on a Pacenti SL23 rim, as have a few other cyclists (the link provides one example).
I believe Mavic Ksyriums are welded rims. If the wheel has a sticker saying "SUP" anywhere, this means it will be welded; the Fitwerks link documents that SUP is a standard Mavic acronym referring to welded rims. Additionally, I have found several links identifying rattles from the sleeves in Mavic wheels, including the Ksyrium Equipe of the late 2000s / early 2010s. The links seem to corroborate that many Mavic rims were welded with a non-structural joining sleeve.
I believe the original poster could epoxy the non-structural sleeve back into place. The Wheelworks site linked previously described one way to do this: drill two small holes into the rim bed straddling the sleeve, inject epoxy through the holes, then put the rim sleeve-side down and let the epoxy cure. This does seem like it could weaken the rim, and I am a bit uncertain about this advice.
In any case, the sleeve in the question above is more accessible. It does seem like the OP could put some epoxy between it and the rim bed, then slide the sleeve back and wait for the epoxy to cure. In some cases, simply adding lubricant has solved the issue; this did the trick on my Pacenti SL23 rims, and it worked for several other consumers. However, the sleeve was in my rim not quite as loose as on the OP's Mavic rim. I could not physically locate it, and I believe it was just rattling in place.
If I am wrong and that rim actually has sleeved construction, then stop riding it immediately. This means a structural sleeve, i.e. the rim isn't welded together, and it's held together by the sleeve plus spoke tension. I'd urge you to verify the rim construction method. If anyone knows that the 2008 Ksyrium Equipe actually is joined by sleeved construction, please comment.
Regarding Astral's assertions about sleeved vs. welded rims, my impression is that most current performance aluminum rims are welded. Thus, the relative advantages between welded and sleeved rims are probably up for debate. I don't think consumers should categorically avoid either welded or sleeved rims. Having the non-structural sleeve come loose should not be a common event, either.