Chevrolet Repair: chevy 4X4 making loud buzzing noise, chevy silverado 4x4, chevy 4x4

Question
we have a 2003 chevy silverado 4X4.  On a recent trip it began making a noise upon acceleration.  We were driving a lot in high tempatures with the ac on.  At first it sounded like it was stuck in low gear and not shifting up, we checked the trans fluid which was a bit low, added some.  The noise has continued and seems to have worsened.  It is a buzzing/loud humming sort of noise that seems to be coming from under the cab/bed.  It starts when the gas is pressed and stops when you take your foot off the gas.  It does not make the noise when reved in park or neutral.  It does seem to be shifting fine as you can feel it, and our mileage has not dropped

Answer
Hi Beth,
Look at the exhaust system, and the tranny housing, and see if any of them are real close to the body or cross members, where the torque of the engine might make them barely touch.
Another buzz I have heard is inside the automatic transmission. A pressure relief valve opening and closing sound.

Van