Handbrake (as in "brake" - slowing something down - , not as in "break" - separate something into pieces) is CPU intensive, and the more one uses CPU, the more power is drawn, and the more power is drawn, the more heat is produced and emitted, and as more heat is emitted, the fans have to spin faster to properly get rid of the heat. As the fans spin faster, they produce more audible noise, thus Handbrake is noise on a notebook.