I hear a tapping noise inside my truck under the dashboard.

When does this tapping noise present itself to you? Do you hear it all the time or just when changing the temp control or when changing from outside air to recirculated air? If it happens during either one of those it is most likely a blend door actuator that is the cause of your problem. Depending on which of those is causing the tapping will tell you which actuator you will need to replace.