i've got this same thing, at about 55 MPH it starts vibrating and humming, and it'll get louder if i go faster and quieter if I slow down, but if i just barely touch the gas pedal it goes away, I haven't pin-pointed it yet, but i noticed if i grab the front driveshaft and twist it back and forth quickly i get a small thud, but i don't think thats abnormal either.