Congratulations and welcome to the forum
Sorry, but I'm not familiar with those diesels, but the description does sound like it could be from the turbo.
Just for clarity, the whine is there all the time at highway speeds on both level road and up grades? It only goes away when going down hill?
Sherm & Terry w/rescues Steve (std Poodle) & Eydie (min Schnauzer)
2015 Jay Flight 27RLS (Camped: 102 nights 2015, 90 nights 2016)
2006 Ford F350 6.0 PSD, Lariat, 4WD, CC, LB, SRW, auto., Camper pkg.