No need to worry, that's basically normal. There are two things to hear:
- Immediately after starting, you will hear the brake booster. This builds up the pressure, so you can also slow down, when you want to. The noise is relatively loud and comes from the back of the car.
- In addition, when you start driving, you hear the self-test of the ABS sensor. This is the soft clack coming from the front of the car.