I have always found getting a friend outside the car helps to diagnos problem sounds. As long as you dont have to drive faster than they can walk/jog it works pretty good, also a little more involved idea is put all the tires up off the ground and see if you still hear it, may not since there is no longer any resistance. However with both methods be very careful
"You can live your life in a thousand ways,
but it all comes down to that single day,
when you realize what you regret,
but you cant relaim and you cant forget."
----Trans Siberian Orchestra