Macbook Pro Retina 15 has a creaking noise even without touching it

These days I found that my macbook pro retina Early 2013 Model has a creaking noise when the CPU and GPU's temperature is going up to 50 degree Celsius, which should be normal temperature when you only run the machine for web browsing and video watching.

And I got this creaking noise sudden without my hand put on anywhere close to the machine, it came out itself. And I have checked that:

1. it is not the sound from the spring on the main board.

2. it is not from the buttom because I tried to removed the buttom and use the machine the noise still there

3. it is from all over the keyboard, most close to the screen.

I am doubting that it is because metal moving while heated.

And I made a video to record it then show it to "Genius" in apple store. But they claimed that they couldn't hear anything, since me and my friend have all comfirmed that it is exsisted!

Watch the video at max volum and at around 3 SECOND and 35 SECOND, there comes the noise.

https://www.dropbox.com/s/o7kqww05d1e2avv/03__35.MOV

See if any one got the same issue.

Thank you and wish you don't suffer from this.