OHxEIGHTxGT

Can anyone help me figure out what this noise is? I hear a high pitched noise that sounds almost like a rapid ticking or like a bad speedometer( had a bad one in my last car and it sounded like it was spooling up). It happans on and off and seems to be more noticeable when it's cold. It only happens when the engine is under load and in gear and specifically at 2,750 RPM(at no other RPM). I though maybe engine or tranny but it's coming from the drivers rear. Anyone have any ideas before I take it to ford? Pleas help!