"Loud tread" noise around 60/65 mph on smooth roads
 

I was driving the GS to the mall today and while I was on the bit of interstate it takes to get there I noticed a noise coming from the rear end of the car. While going about 60/65 mph I heard a noise that almost sounds like the tread of a tire that's very rough going over the road. This noise would happen continuously and it sounded in sync with the wheels turning. I don't hear it in lower speeds and I notice it mainly on smoother-paved roads. My father thinks it's the tread of the tires but I have a friend that thinks it might be a slight alignment issue. I'm learning towards alignment. It's a slight vibration but not enough to really feel it in the steering wheel at all.

Any thoughts on this matter are appreciated and I'm sorry I couldn't describe the noise any better, if theres any confusion on that I was try my best to elaborate further.

Ball joints!!!