2003 Cavalier LS Sport Coupe (Road Noise)

I recently bouht this car. It runs and rides nice, but it has a tone of road noise. I got hold of some used cheap tires, and made the swap to see if there would be any difference, but no. The faster the car is going, the louder the noise. At 60mpr, I can push the clutch in, and be coasting, and the noise is still there, so it is definately not the engine.
I don't want to go out and buy expensive tires, just to find out that they make no difference. The tire size is P205/55R-16.
The noise seems to be loder in the rear of the car. Could the ground effects of the sport coupe be the cause?
Has anyone else had this problem?

is it stock?

Yes. It was in a front end accident, and has new hood, grill, etc. But everything runs great. Just seems to be a lot of tire noise, unless the back axle is making it (I doubt it).
These tires are wider than the base model cavalier.

there souldn't be that much in the rear. You could always put some soudn deadner in the truck