Was Diff Whine - '72 280SE 3.5
To all that replied to the original post over the last couple of days, thanks - however ...
I feel a little foolish here guys, but I went out with my teenage boys in the car tonight and asked them to listen to the whine.
The consensus is that the whine seems to be coming from below the tunnel, pretty much under the a/c blower area. Unless diff whine can be referred to this area, I am feeling like a a bit of a dill (that's Australian for an idiot!).
I will still check the diff when I have the car safety checked in two days time before registering it in my state (I bought it just across the border) - but does my revised location for the noise source suggest any other causes.
Again thanks for the advice and sorry for the poor diagnostics on my part.