If you recently R&Rd your Truck Battery... the tap-tap-tapping sound will be coming out from under the Dashboard involves the HVAC Blend Door Actuators trying to re-learn their positions and sorting themselves out. This action can also take place when they are failing as well.
First chance you get... Visit any of
@Mooseman 's Posts ...and view his "Cygil" or Signature Line at the bottom of these postings and follow his instructions on "How To Download Your Service Manual(s)". Having the Trailblazer/Envoy "Shop Bible" at your beck and call will allow you to search out the Chapters and Verses needed to sort of get grounded in the necessary reference information before hand... and in addition to GMTN Members' Helpful Hints... it will make the OEM Repair(s) easier to perform.