When I'm on the freeway anywhere from 55 to 80 mph when I give some light throttle (transmission stays in overdrive and does not downshift) I can hear a rotational sound almost like something is out of round. It's not a grinding or howling sound. I can definitely hear it and feel it in the accelerator pedal. When I let off the accelerator it goes away and does not make the sound at a steady speed. Any ideas?