Hrm.. With the daul turbo set up that we have on these trucks, some times both turbos are not really working. The VGT turbo usually does the work at lower RPM's and the other one starts working when the VGT starts needing more air. It sort of pre-pressurizes the air a bit before it goes to the VGT to be pumped up to a higher pressure. I am guessing that at the RPM/speed that you mentioned you are starting to hear the two turbos together. Might be a harmonic interference between the two spinning turbos making the sound you are hearing. Don't know.. But either way, if it did it without the drivetrain moving (brake torquing) then it isn't your driveshaft.