WTF is in the frame rail?

[singlespeed]

I noticed a plastic bag in the each side of the car in the frame rail. It appears to be full of some kind of beads. Why are there in there? It looks like something that would have been there since the manufacturing process. Hard to see, but look for plastic in the center of the pics.

[team_v]

Probably sound deadening or crash protection.
I have it in my car too, seems pretty common.