I opened the access panel that is near the floor, just behind the driver's seat on a 65 Econoline van, and found it stuffed with fluffy white stuff... it looks a lot like blown-in attic insulation.

Is this supposed to be there?

I found the same kind of material between inner and outer rocker panels on a 1970 Cougar, and a bunch of Cougar guys said they had never seen it before, and it must be the work of rodents.

Seeing it in a similar place on a different vehicle makes me think maybe it's supposed to be there.

It looks like it would trap moisture and promote rust. Is it just sound deadening material?