[russv]

I went out and did the 35 mph and 70 mph sound test and noted the sound described at both speeds. It seems to come from the rear of the car. I think there are 3 possible theories:

1) The tires create the harmonic which is then amplified through the body of the car.
2) The exhaust creates the harmonic
3) The rear differential creates the harmonic

I'm tending to rule out the tires as the harmonics are at differing speeds. (My opinion, I could be wrong).

Having owned rear wheel drive cars in the past I have experienced drive train noise. Consider that you are dealing with 3 components: transmission, drive shaft, and differential.
I've also experienced cars with exhaust systems that demonstrate a harmonic sound, but it is usually over a certain RPM range and not necessarily speed related (Mini Cooper, various Hondas).

My money is on the drive train, probably drive shaft/differential. That said I am not too concerned at this point, unless it gets worse. Probably a normal sound.

AND, let's keep the discussion civil because it appears to be endemic to all our cars and is a concern to some.