The things that I can think of that would sound like a growling noise are the wheel bearings, or a hole in the exhaust.

Other than the sound, does the car drive normally? Is the gas mileage normal?

Is the sound there when the car is coasting? How about throwing it into neutral while accelerating and checking for the noise.

 

Also, does the sound change when turning left or right?

Is it possible that there are 2 sounds, 1 at idle plus a wheel bearing when moving?

 

According to her, the gas mileage is much lower than it should be. I don't have the stats to back this up or explain just how bad it is, so it is hard to say.

Probably not the wheel bearings, since revving the car higher makes the noise too. Hole in the exhaust...hmmm...something to check on for sure. I'll see what I can do about checking that one out.