2003 Chevrolet TrailBlazer EXT knocking sound



There is a knocking in the very back of my Trailblazer. It only happens when I first start the car. Goes away within a few sec/min. What is it and how do I fix it


Most likely a rear a/c mode or blend door actuator. Sometimes on this generation of GM trucks disconnecting the battery for a period of time will fix the problem, as the computer will recalibrate the doors operation.