Hissing noise

Hi I just recently bought my car and realized that when I apply the brakes theirs a hissing sound from inside the car. Now I did a bit of research and loads of people say that its a bad brake booster. I have driven the car around 4-5 times since I noticed the problem but the brakes seem absolutely fine. Now is this normal or should I book it in with a mechanic. Bearing in mind that the car just went in for a service