There aren't many components of a wheel that can come loose to the point of rattling.
I suspect it's the little aluminum sleeve inside the rim at the SUP welded seam that has come loose. It's not an uncommon problem with older Mavic Open Pros. (Which weren't around 25 years ago - they're probably more like 10 years old.) You can try "gluing" the sleeve in place by squirting a bunch of watery glue (like Superglue) into the rim through the valve hole and then waiting for it to run down to the opposite side and harden, but most of the people I know who have run into this have either just ignored it (it's only noticeable when the wheel isn't rotating) or replaced the rim with a completely new one.