The sound that you're describing (if I'm understanding you correctly) makes me think that it's probably the "pinion bearing", since you only hear it while accelerating or as you're driving normally. But as soon as you let off the gas, it goes away.
For those of you who don't know what a pinion gear is, it sort of looks like a giant drill bit that a dentist might use to grind out tooth decay. This gear meshes with the ring gear and spins the axle.
The pinion gear is set horizontally "beside" the ring gear, and when your accelerating, it actually wants to "drive" itself forward as if it were "drilling" or penetrating ahead, and when you let off of the gas, it "relaxes" and pulls back so to speak. If the pinion bearing is faulty, or worn, it allows the pinion shaft to travel (ever so slightly) but enough to make the gears "whine" or "hum". Since the alignment of the gears is so critical when "setting up" a differential, any movement in this area, would cause this misalignment to occur.
I'm not sure, but I would think this problem pops up more often with manual transmission cars than automatics simply because the shifts of an auto are more "seamless" than those of a stick shift, as are the launches. But that's normal wear and tear. If it's a faulty pinion bearing to start with, then the end result would be the same in both automatic and manual cars.
That's what your complaint sounds like to me, and if in fact that's what it is, then the 7/70 warranty should cover it.