The night was getting deeper and the surrounding was getting silent. Neighbors were turning off their television sets, radios, computer. They were going to sleep. The same was happening to our home. However, I have the habit of staying awake even if most people around me are sound asleep.
The surrounding was silent. I was hearing clearly the wall clock ticking sound, including a strange low knocking sound which I could not determine the source. I was sure it was somewhere near my seat. I have been hearing it and looking for it for several nights.
Accidental discovery. I filled my glass with drinking water from the Tang litro pack. After, I saw the flip flop cover popping up accompanied with the low knocking sound. So this PET bottle was the culprit. It was the thing capable of making low noise by itself. It was popping about every five seconds.
I am wondering why. I have the idea, PET bottles reacts with its contents with repeated use. The force that was making the flip cover open partially and make a knocking sound was a gas build up. The easiest scape route was the loose flip cover. The idea of PET polymer reacting to water and other foods is acceptable to me but the gas production is simply unbelievable.