The sounds from plastic bags are generated mostly by you, because the bag will not rustle by itself.
The noise is caused by sharp movements of the kind you get when a stiff plate buckles or gets rubbed.
The bags are made of polyethylene film which, untreated, is waxy and floppy and not very noisy. It is elastic rather than plastic, so it absorbs stresses quietly.
However, to make the bags, the film is stretched to get it thin enough to be convenient to handle and cheap enough to give away with the goods.
This partly aligns its molecules into stiffer sheets.
To make the bags look better and the contents more anonymous, manufacturers add fillers for coloring and further stiffening.
The result is a bag which audibly protests every crinkle, crumple and abrasion.
