I had a thermos flask that I had never used since I had bought it. Today, I decided to take it out and use it for storing hot water, so that I don't have to heat water every time I feel thirsty. An interesting incident occurred. I heated water to near about boiling temperature, and filled the thermos flask with it. Then I tightened the lid. And then I started to hear this sound from the flask. I realised that the sound is coming only if I put in hot water, not cold water. It's a weird sound. It is stopping if the flask is kept undisturbed for some time, but starting again if I move the flask. Can anyone account for this sound? The room temperature is about 15°C.