Consider a plastic soft drinks bottle. Shake it up so that it fizzes and pressurizes the environment inside the bottle. Tap the side of the bottle, and it'll make a low bing sound.
Now open the bottle and release the pressure, so the inside and outside equalize. Tap the side again - now it makes a hollow thunk.
Why does the bottle resonate like this when it's pressurized, and doesn't when it isn't?