Wind/rattling sound near B Pillar

It's usually the undershield making noise. The plastic retainers are garbage and come loose. Stainless M6 nuts with fender washers to replace those will solve that once and for all.

I'm talking the cover underneath the car. At speed, the noise will seem to come from the door or the B-pillar but it's actually the undershield that flapping around.

Just fixed this myself after chasing the high speed fluttering noise. Some 1" fender washer on top of a M6 nut to replace the plastic retainer will fix it. I'm in Texas so I just used regular steel washers. Being in snow and salt country I would recommend you use stainless hardware. Ford uses essentially the same thing for several of the attachment points. It's another Ford engineering mystery as to why they use the crappy plastic fastener on some points and more robust nut and washer other places.

Hi, I have the same issue. How did you remove the plastic retainer? Is the center post threaded to accept the M6 nut?

Anyone else experience a rattling sound coming near the B Pillar when driving highway speeds around 50mph and above? Brought mine to the dealer this past week, and they installed a new A and B pillar trim, but the sound still persists. Any thoughts? Thanks!

Thank you all!!! I would have never guessed it, but this fixed the issue. Both driver and passenger. I tried water test, tension test on door and windows, and nothing helped identify the issue. I was 100% confident it was the black pillar plastic (keypad) that was the issue.
Found this thread, checked the underbody, and found a number of loose plastic nuts and missing nuts on both driver and passenger sides.
For others trying to resolve the issue, it’s a very inexpensive fix and definitely worth the try. If you can find M6 nuts (not bolts) in a hardware store without help, you can do this!!
Start by using your hands to press the fiber underbody (feels almost like cardboard egg carton). There’s probably 10-12 connection points on passenger and less on driver (maybe 8-10). On the outside edge, below the front and rear doors, is where the crappy plastic nuts are located. It should be loose in this area (push fiber underbody up - any loose is too loose)!
Rip’em out! Pic below shows you can use small flatheads or long thin needle noise to release the clip, but it’s more enjoyable ripping out.
Get M6 nuts and big washers (pics below from Home Depot). I used 10 of each. Replace plastic nut with washer and nut. Use 10mm socket or hand and tighten. Repeat on both sides.
I checked all metal nuts and they were secure. All but 2 plastic nuts were VERY loose.2 completely missing.
Another pic of tools for the job. And don’t forget a nice piece of cardboard to lay on.
Test drive up to 65 mph and total silence afterwards.
Thanks again and good luck to others.

ps yes I’m getting galvanized or stainless eventually. Zinc was available and less than $10 total.