unexplained noise

This really freaked me out a while ago. I was eating tortillas with salsa. The salsa was in a one of those portable containers that chi chi's has. Basically it's just a plastic container with an aluminum seal glued to the top.

I pulled the lead back and held the cup in my hand, it made a high speed metallic popping noise. I sat it down on my table and it stopped a few seconds later. After many "experiments" I came to the hypothesis that the vibrations of my hand shook the connecting point of the aluminum, so it hit the plastic cup.

The thing is I bent it back and it stopped, nothing I do can restore it to its magical vibration point:( Yeah, I have too much time on my hands...