I have a brand new 2.7 with 4k on the clock and noticed the same thing the other day. Usually when I am at around a solid 60-68 mph and 10th gear. Gonna ask the dealer about it.

 

Exact same issue with my 2000 mile 2.7 ccsb. It was hard reading that 62 page thread of bull****. I actually believe it's pinion angle related. It only happens between like 50 and 65 mph while at tip in. This means 8, 9, 10th gears mostly, but I did lock those out and found it still seemed to be an issue in 6th. The springs are impressively small in this truck and Id imagine we get more axle wrap than previous years with 2 and 3 springs to a pack. I think to compensate for this, Ford engineering likely set the optimal angle of the pinion to account for higher load situations than tip in. I have no noise at all off throttle and coasting. 65+ is glass smooth. My pinion angle delta is like 2.5-3 degrees of difference pointing downward as stock.

I actually just tried stretching and cleaning my coils to see if that theory worked, but no luck. 91 vs 85 (colorado) didn't do anything for me either. I first noticed it when I removed the rear blocks, but now after reinstalling, it still seems to be present.

 

Had my dealer appointment for diagnosis.
Summary can be found in Post # 631 on this (very!) long thread:

https://www.f150forum.com/f129/shudd...ispreloading=1