Everytime i start my c180 kompressor i get an annoying knocking sound from behind my dash for about 20secs then it goes what could this be?

 

What body style W203 or W202 , if comming from dash area and is a w203(2000-2005 yr.) chances ae the a/c flap swtepper motor actuator arm is cracked and needs replacement. remove your radio and a/c pushbutton assembly youll see 3 roundish stepper motors turn key off wait 10min turn on place hands on the motors (should be 3 in a row) noise probably comming from left motor next to ignition switch. can be removed with a ball t25 torx .also check by a/c on and change the vent settings from face/floor to defrostthe sound may be duplicated or could be a leaking vacc pod element on the W202 chassiswhich is tricky to diag. best to go to shop