Beep alarm when using my Macbook Pro 15" Early 2011

Good morning everyone,

first of all I apologise for any linguistic error I'll do, being a not native english speaker.

My problem is the following one :

I have a Macbook Pro 15" (early 2011), and I installed last fall a partition with Windows 7 by using Bootcamp.

All of a sudden, one day my Mac started to not working properly: a three beep sound repeated avoid me to use properly it because when it started doing those beep it restarted.

Thus, I thought that everything was caused by windows 7 partition and I decided to delete it and format my hard disk.

When it was formatted I reinstalled the new OS X El Capitan and it seemed everything was fixed for some days.

One week after, more or less, my Mac started to emit those beep. These beeps doesn't occur at a specific moment (in fact the computer starts normally), but it seems that it occurs at a random time when I am using it.

I tried to use Hardware diagnosis in order to check the RAM but everything is OK, also HD was checked and it was OK too.

I would like to know how to proceed and fix this problem. The sound is a repeated set of three "beeps" that occurs at an apparently random time.

Is there anyone with the same problem or anyone so kind to help me ?

Thanks

Sorry for english