Bad service

My wife took my Everest to check for a rattling sound underneath the front of the vehicle which was just serviced prior to the rattle sound which they did not pick up during the service. The lady who assisted my wife was very unprofessional and rude. They made my wife pay R546 for two pop rivters on oil cooler heat shield, although my car is still under warranty. Don't u der stand how this could happen a d how is this my responsibility to pay. This was a very bad experience and I will not take my car to Menlyn Ford ever again