Growling noise

Have a '97 Dodge Stratus that has just started making a growling noise. It starts at about 20 mph & seems to get louder as you increase speed. Shift into neutral & the sound is still there. In neutral, with car stopped, engine sounds normal reving it to high RPM. Transmission shifts fine. Does anybody have any input as to what my problem might be.

I have a 2000 stratus and had noticed that when I moved the rear tires to the front it sounded bad.This was caused by feathering of the rear tires due to the toe-in/toe-out being incorrect on the rear.The rear tires had been worn rolling a bit sideways but when placed on the front the noise got worse due to them rolling against their worn tread pattern.

If it's consistant w/ the speed, it's tire related (as jlsumme stated) or maybe the bearings.