It should be that the vacuum layer of the inner container of the thermos bottle is broken or damaged, and the insulation layer of the inner wall falls off when shaking.

The noise in the bottle liner is the reverberation formed by multiple reflections of external sound waves after entering the bottle liner. A good bottle liner, because of its high smoothness and high vacuum between layers, has a strong effect of reflecting sound waves, and the sound waves are not easy to spread away, so its reverberation is great.

The thermos needs to be filled with half a bottle of water for the first time, and then add 1 to 2 tablespoons of salt. After tightening the bottle cap, shake the water bottle up and down several times, and then pour the salt water into it.