When switching to A4WD in my 2003 TB EXT I'm hearing a banging noise from underneath the car. It feels like one is going over speed bump at high speed (like when one is approaching a toll both). I had the front differential replaced last year at the dealer. The original had an seal leak that could not be fixed. Does anyone know what is happening or perhaps guide me on how to troubleshoot this problem. No noise or problem in 2WD. Thanks.