Why do potato chip bags make noise?

Its because the weak bonds between the polymer molecules break due to stress and the resultant energy is released as noise..(the stronger bonds keep the plastic together so you dont have to worry about crumpling your bag to dust with a great outburst of noise!)

Basically 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.

Due to the crystallization of the polymer made to make the plastic stronger. Old plastic bags from your grocery store used to stretch and rip, but now that they are crystallized and they don't stretch as much any more.

The result is a bag which audibly protests every crinkle, crumple and abrasion.