Maybe there are exotic bugs in there! Still got the bug vacuum?
The only other thing I can think of is that maybe, just maybe there are teeny weeny birdies living inside pop cans that make the can sound like it is chirping when it really isn't the can at all.
Only on the DIS, perhaps only on an Iwas thread, would I be typing that sentence.
Last time I heard a chirping can it was because my <insert sarcasm here> lovely older brother decided to put crickets in 2 beer cans in his collection that was still in my room after a room switch. I ended up spoiling his fun by finding them and shaking them out of the cans while holding them outside my window.