The Mystery of the "Tok... Tok... Tok..." Sound

It was 2+am in the middle of the night. A middle-aged lady security guard was walking her rounds around the various blocks in Hall 11 as usual. Although the corridor was brightly lit up, it was very quiet as most of the residents are asleep. She could even hear the sound of her footsteps.

 

In the midst of her rounds, she heard a series of strange sounds:

 

"Tok... Tok... Tok..."  Around 10 second later, the same series of strange sounds. "Tok... Tok... Tok..."

 

Curious, she walked towards the source of the strange noise. It was a room, and the noise is the sound of something hitting against the door. Without further thoughts, she knocked on the door. A young man opened the door to greet her. She asked,

 

"Why are you knocking on the door in the middle of the night? It is very noisy leh. I can hear you from far away."

 

The young man replied disgruntedly, "Sorry aunty, disturbed you ah?"

 

Trying to find out what was happening inside the room, the lady security guard continued to probe, "Why you knocking on the door? Very noisy leh."

 

However, the young man ignored her question and said,"Ok ok." Then he closed the door. It was obvious that he was trying to hide something.

 

Yet, the lady security guard could not to do anything else. With her questions unanswered, she could only continue to make her rounds. Still, at the back of her mind, she simply couldn't help thinking what was happening behind that door. Why would anyone knock on the door in the middle of the night??? However, after a while, she resigned to the fact that some questions in the world can never be answered, some mysteries in the world can never be solved. And she went back to her mundane task of patrolling the Hall.

 

Little did the lady security guard know that, a while after she left, the strange errie sound began again...

 

"Tok... Tok... Tok..."

 

 

(This is not based on a true story. This is a true story. Can anyone solve the mystery?)