Adding all the foam did not work for the high speed rattle, but the car is now as quiet as a mouse over the 1000 city bumps I go over on a daily. This is definitely an engineering design issue, theses parts should've been lined with foam to stop rattles and the plastic screw caps same thing, should be lined with foam (electric tape does the same). So I started exploring rattle forums under the body of the vehicle as
@Cdubya suggested and jacked up one side of the vehicle on stands. I found the following:
1. passenger side under body cover (covered in multiple threads) had a bolt that was threaded 2 times near the rear wheel, took about 16 turns to bolt it down, I'll take photos later. This lose bolt would have cause the body cover to flap at high speeds and hit the the mid pipe muffler but was not necessarily the sound that was bothering me. This is the second bolt I have found just literally hanging by a thread, the quality control group and the assemblers are lazy turds or pissed at the vehicle because they can't afford it. I did not have any of these issues in my 2020 Ford Edge ST.
2. The passenger side under body cover cover bolts were also unusually lose, contributing to a flapping under body, about 2 turns to tighten each one.
3. The passenger side clip (just one), was also popped off, I will take off and replace with metal bolt when I have time. I pushed it back in for now.
4. All the heat shield bolts were not hand tight, each one could take 1.5 turns until it was torqued on to hand tight, which could contribute to rattling at speed.
5. There is another heat shield on top of the muffler, same thing, those bolts took another 1.5 turns until it was torqued on to hand tight.
6. Most importantly.... There are two overlapping pieces of heat shield in the center of the vehicle (where the rear chairs are) that are not bolted to the body, but at high speeds would definitely tap and occurred "exactly" where the noise was coming from in the cabin. These overlapping pieces have a slight gap between the body to the shield and to the overlapping shield. I utilizes 3m high heat, waterproof all weather double sided sticky to solve it. Since the gap was slightly larger, I double stacked each 3m sticky between the heat shields and where the shield meets the body.
I will take photos of everything and upload later.
Took the vehicle on a road test thing morning, 65, 70, 75, 80, 85, noise solved!
I spent a total of 20 mins tightening bolts and adding the 3m foam under the vehicle.
The Explorer ST is definitely not a starter car if you can't hunt down rattles or issues with some basic tools.
https://www.amazon.com/gp/product/B01BU7038A/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
These work on mounting auto parts such as HIDs and work for years,e.g. even after 8 years of mounting through summer heat, snow, you still have to pull and clean it off for it not to stick.