The lids on the pickle jars I have at home are made of metal, probably painted tin or steel that have been coated with plastic on the under side (including the screw threads). The noise you hear is from a small "cap" in the middle of the lid, about the size of a nickel. A vacuum is pulled on the jar during the processing and the "cap" pops out when the lid is unscrewed an air enters the jar, hence the popping sound. In fact, on the jars I have at home, there is a note on top of the lid to the effect that "inspection of this lid will confirm that the contents are fresh". You can feel if the round circle of a cap is recessed or has popped up. If it is up you know the vacuum is no more and the pickles should not be bought or eaten because they are not fresh.
