what would cause my car to suddenly start sounding like i had installed an exhaust system?

I had driven nearly 500 miles over the course of 3 days. The day it started I had drivin about 160 miles and then stopped the car for a few hours. After I got back on the road, I noticed that when I hit 20 mph the grinding or growling or like something was rubbing causing vibrations sound. It vibrates betweens 15 and 70 mph but at 70 it almosts stops. It doesn't make any sound as I turn corners or while I'm sitting in idle.