Since the noise goes away when you downshift, I assume it is related to RPM, not vehicle speed.
Do you hear the noise when in 4th or 5th at the same engine RPM as in 6th? I'm thinking at 72-78 MPH you are seeing 1500/1700 in 6th?
I tend to not run steady at these lower RPM's. Prefer to stay "ahead of the curve". This normally has me using 6th very little as I'm running 85 or so before I get there.
If the only time you are hearing the noise is in 6th (not 4th or 5th) at low RPM it could be a result of wear on the gear or sync ring, but more likely a harmonic sound because of the drive train being in a "relaxed" situation with the higher ratio.
I wouldn't worry about it a whole lot as long as there is no vibration. How many miles on your Z?
