Im just after pouring myself a cup of tea in a standard mug that I've used many times before. After pouring the tea I sat down at my desk and heard a wierd sort of purring/croaking sounds, a search later made me realise it was coming from my cup of tea and it actually sounded like a little creature was in my cup of a metal ball or something was spinning around in it. What causes this noise? Is it just the cup in the heat of the tea? And why have i never heard this before?