I noticed a growling sound in my steering wheel and took it to the dealership. They said there was a lot of play in my steering wheel and had to replace the rack and pinion. I just got it back yesterday and it's fixed. I have had a car that made that sound while driving and it was actually my wheel bearing. I know that's not much help but wanted to share my experience. The rack and pinion was only when turning, Wheel bearing was anytime the car was moving.
2010 G6 GT Black 41,000 miles. Under GMPP Major Guard warranty until 112,000 miles.