Strange sound

I hope someone would help me with this concern.

I have recently bought my macbook pro in Brussels. In the first days and weeks all seemed normal. However, for the last two weeks or something, I have from time to time a sound, like a notification, made out of the mac. I see no other "symptom", How could I understand what it is about?

I am worried that it might be an infection!? I have an ESET anti-virus product for Mac...

Many thanks for your suggestions!?