2020 Silverado RST with 14500 miles, bought new last April. The blend door makes an audible noise when opening and closing. It happens at stop signs when engine shuts down and when pressing the recirc button. Best description of the sound is similar to imagining a golf ball in the vent 'thunking' each time the blend door opens/closes. GM tech said its normal and all trucks make that noise, and there is nothing to be done. Can anyone out there confirm you hear a knock or thunk every time the blend door operates?
