1993 Suburban C1500, 5.7L, 2WD, 3.73 G80 rear diff, auto tranny (4L60E), 135K miles.
There is a distinct whining noise between about 40 and 55 mph when load is on the drivetrain. If I take my foot off the accelerator and coast, the noise goes away. Again, it starts around 40 mph and goes away...