my 2 cents
at 900 miles - the gears should have 'mated' to each other. Was part of the deal to have the fluid drained after break-in? If so, when the cover is removed the wear pattern showing on the gear will tell about installation. Has the noise been present the whole time and at the same speed/load?
I had a set installed by a friend, also FRP-3.73's, they are set a little tight, and mine whine a bit on coast, around 60 mph. They have quited down over the past 4 months (or I just dont notice the whine at this point). This is one more (good) reason for me to keep it under 100 MPH though.
96 GT - PI Intake & cams, O/R h-pipe, Flowmasters - 40s, Short Throw Shifter, 3.73 FRPP gears.