Definitely the wastegates. Mine do it not only on cold startup, but even warm start ups and I can generally hear them anytime I drive through something like a "drivethru" where there are barriers on the sides that reflect the sound. I was able to sit in a dealership bay, put the car in drive, and rev it to about 2700 RPM, and it made that rattle continuously from both turbos. I've been fight Ford for almost 9 months to replace the turbos. They say its normal, but I disagree on several fronts. If it didn't do it until the vehicle hit about 7k miles, then it's not entirely normal. If a brand new one sitting next to it doesn't do it, hum? And I've built other turbo engines with electronic and mechanical wastegates that didn't make that kind of a noise. Metal flapping on metal that much will eventually wear out or cause another issue.
It's super annoying to me, sounds like a piece of crap 70's Datsun with a loose exhaust when I'm cruising through parking lots etc. For your edumacation, before Ford would approve anything, they required my local dealer to the do the VCT fix even though both myself and the dealer master tech said the sound was coming from the wheel wells and under the car, not in the vicinity of the chain/vct area. How stupid to tear apart a car for a full week when we all know that's not the noise.
I have exactly the same diagnosis and concerns you noted above. I have 33k the truck and the cam phasers and VCT solenoids have been replaced. The rattle remains cold or hot, but louder in the first 2m during startup. I also had both rear taillights replaced, due to moisture buildup. The new taillights do the same thing, and they aren't as tight to the body as the originals. If to do over, I wouldn't of pushed to have them replaced.