You still have one more to change. There are several youtube videos that shows you how to changeout the actuator on the left side, not accessible through the glovebox. It is very awkward to change, but can be done with patience. If this helps, do not forget to vote best answer.

Try this link: www.youtube.com/watch?v=8FYzK-kS-Ww

Hello jeffrey,

We are sorry to hear you are experiencing concerns with a knocking noise in your vehicle. We'd be happy to look further into this concern and reach out to your dealership. Please feel free to reach out to us directly of via email at socialmedia@gm.com if further assistance is needed.

Thank you,

Andraya

GM Customer Care

Are heater air-doors vacuum controlled?Could be a slowly leaking vacuum line, w/ negative pressure hovering around actuation threshold. Or, try asking "Who's there?"

Someone is having sex.

I have a knocking in my dash sounds like it's directly behind my radio. It's a 08 chevy impala with duel climate control. I have already have replaced both actuators behind the glove box. I went to re - replace one and it started knocking again with the actuator unplugged. It randomly does it. It will do it just sitting in the driveway and it's off locked up and hasn't been touched in 15 min. Or while going down the road.