The wife has a 2011 Expedition and this week I noticed a knocking noise. When it starts, there is about 7-8 knocks coming from the passenger side about even with the third row seats. It goes away after about 10 seconds but it seems ominous to problems ahead.

Thoughts?

When you start the engine you hear knocking at the back of the vehicle?

Yep, back even with the third row on the passenger side. I did some googling and apparently that is where the rear AC/heating stuff is.

Well, thought you meant engine knocking at first and coming from back? Huh?

But if you say this is where AC/Heating stuff is, turn rear fan motor OFF and see if if it still makes noise. Could it be the fan motor spinning up when starting vehicle? That's all I got based on what you mentioned.

Rear blend door actuator

I bet the sound goes away if you turn off the heat or air to the rear seats. It may also go away as the temperatures even out inside the vehicle. I replaced the part in my expedition (not difficult, but did require a fair bit of interior removal to get to it. Unfortunately, it solved the problem for about a week before the new part failed. Now I just live with the knocking sound sometimes if I have kids in the back seat and turn on the heat/AC for back there. Not worth it to me to take everything apart again.

^^^^

What he said.

REmove the panels and the actuator is right there.

Under 100 bucks and less than an hour to DIY swap.