Imac 21.5 mid 2011 loud noise when starting

Hello everyone,

the noise is very loud and it sounds like two separate shots from a machine gun. It's even scary if you have the speakers on. After starting it runs smoothly as it always did, and i run Logic Pro on it intensively, it shows no problem whatsoever with the hard disk nor with the 4 RAMs.

Here it is:

https://www.youtube.com/watch?v=en4Rmb1a78I

I have attached a video of the sound.

Can anyone help please?