Residual pocket of carbon from the remaining evaporating droplets inside the can, reacting with the aluminum and forming small amounts of gas. The gas pushing out between the slightly viscous sugar fluid and the side of the can, causes minor but audible vibrations, sounding like a rapidly ticking wrist watch.
Could also be a bug! If the scritching was as loud and erratic as described here.
Yeah, but the inside of an aluminum can is about as unnatural as it gets. It's a common element, but doesn't occur in raw deposits because it reacts very well (the reason is doesn't rust is because it instantly rusts... it forms a complete aluminum oxide layer immediately on contact with air that prevents further degradation). It's also a pain to refine... it takes a catalyst and a lot of power. Then there's the technology of making the can... a lot of resources have been thrown at developing the aluminum can.
