99gmcyukon

I need help diagnosing a problem. I have a loud whirring sound in my 1999 GMC Yukon. It only happens when I am going over 60 MPH and when I press the gas pedal. If I am going over 60MPH and let off the gas pedal the noise stops. It can't be the tires because it only happens if I am pressing the gas. My mechanic thought it was a U-joint (whatever that is) and replaced it, but that did not fix the problem. It this the drive train or transmission? Can anyone help?