Dani,
Actually some airliners now are louder on the approach than on take-off.
Modern high by-pass turbofans can be incredibly quiet. One of the major sources of noise is now the airframe. In landing mode flaps/slats are extended further creating greater disturbance in the airflow thus creating more noise.
Also departing aircraft clean-up quickly ie retracting the gear immediately after rotation, whereas landing aircraft have the gear down for a long time. The gear creates a LOT of airframe noise.