Wow guys.................that's not a good thing at all. It is turbo bark as stated above and not cool. Stupid people will tell you it's ok and just the turbo spooling down, which it is not. First off, that's what a blow off valve is for. BUT, diesels don't produce vaccum so they don't have BOV's. Spend about $1k and you can get one. That being said, the noise you hear is the turbines in the turbo fighting the boost pressure after boost is reached and it has nowhere to go b/c the vanes closed as you let off the throttle (or wastegate if it applies, but not in this case). You should always roll into and out of the throttle if you're beating on it, and this is the reason.
Do a search for the term "juicegrips". To me that sound under normal driving is a sign that the truck was abused. Too much of it and it happens more and more often at lower and lower boost #'s. The result is a blown up turbo..........a weak point in the 6.0L as it is.
On a similar note, if this is "normal" according to Ford, no wonder the 6.0's have turbo as well as top end problems. Excess cylinder pressure could just as well be caused by turbo bark too IMHO.