There is a campaign for the AC fan assembly. Check with your dealer if it applies. They will need your VIN number to run it and to check to se if campaign is open for your truck.

Check that your radiator fans are working. what is happening is that when you are stopped, your pump is having a pressure build up since the freon isn't being cooled and possibly the pump will stop spinning and start to screech because of the pressure. When you are moving the fans don't need to run and the freon is being cooled and so everything is working normally. Your fans should turn on while sitting still for a few minutes. If your engine temp is over half, they should be on. Let me know if you have any other questions with this