Popping Noise from Rear A/C
On my 2003 Expedition I just replaced /repaired the Rear A/C actuators. There are 2. One is top mounted and one is side mounted. They are in the passenger side, rear panel next to the third row seat. The clicking noise is caused by a stripped gear in the actuator (gear box), which is easily removed and replaced.
In my case, I replaced the top one, the air distribution actuator, which had a stripped gear in it, only to find that the original clicking sound came from the side mounted one, the Temperature Blend Door Actuator. Consequently I then replaced (repaired) the side mounted one.
Lucky for me, I opened each actuator case and saw the stripped gear in each. In the side mounted actuator, I found that the stripped gear was the smaller of the 2 gears. It happened that this smaller gear matched the smaller gear from the top mounted actuator (which was not stripped) so I swapped the good one for the bad one and fixed two actuators by only buying one replacement.
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