I hear weird sound while I am using my iPhone 6 plus, what could it be?

Hi!

So, when I use camera app on my iPhone 6 plus, I hear weird sound. I hear it when I use Snapchat and Retrica too.

I don't hear it when I use front camera!

The sound can be heard on recorded videos as well, so I recorded a video and uploaded it on Youtube.

You can hear it here:

https://www.youtube.com/watch?v=N6vJQBibxgA

I bought this phone 4 months ago and it is still like new. I don't know why this happened and I didn't hear this sound since yesterday.

I tried to restore my phone and it didn't help.

Does anyone know what is it?

Thank you very much.