Why do marbles fall at night?

In the past, people lived upstairs and hated people throwing steel balls or smashing things on the floor. Sooner or later, when the residents upstairs move out, it will be quiet. However, these voices are still there. Are you scared?

In fact, this phenomenon is very common. Conditional people can do experiments and let others throw marbles at your roof. If you don't listen carefully, you can't hear! In fact, the floor was so thick that a small marble fell to the ground that people downstairs could not hear it at all.

We often hear people throwing marbles upstairs. Because the floor is supported by steel bars, under the action of weight, steel bars will produce bending stress, and there will be light pipes on the ceiling, and mold will enter along these gaps, especially where steel bars contact with cement, which is their ideal living place. After a long time, the mold will erode the cement around the steel bar. With enough space, the steel bar will move back and forth under the action of cold and heat, which sounds like the sound of marbles falling to the ground.

Then, the roof heats up and cools down, and the insulation material shrinks. This building is a big loudspeaker.

Or which part, with other sound sources.