A new popping sound

Last year I heard this interesting popping sound. Now there is a different popping sound. It happens when I remove the canning jars from the canner. Sometimes it happens as soon as the canner lid comes off. They are fun little sounds, one for each jar. As the jar cools the contents begin to contract, the metal lids "pop" as they suck down on the jar forming a vacuum.
Right now I am canning tomatoes, we will soon be pickling brussels sprouts, and making apple sauce and apple cider. There will be a lot of popping going on in our house. A wonderful sound of harvest season.