Low Howling Noise Under Santa Fe
 

Just yesterday I was driving our 2004 Hyundai Santa Fe (Allwheel drive 3.5L v6) and it started making a low kind of howling noise.
-It only makes the noise when the vehicle is in drive, and moving forward.
-At higher speeds the noise is quieter but that could just be from the road noise.
-It doesnt make any noise in reverse.
-The noise will stop as soon as the vehicle comes to a complete stop. Even in drive.
-It sounds like the noise is coming from underneath the center of the vehicle. You can kind of feel a vibrating with it when your driving.
-It is a constant kind of noise, not a ticking or anything like that.

Anybody have any ideas?
Thanks!