2007 Sierra SLE 4X4 noise
 

I am having problems w/ my 4-wheel drive making humming noise when it is in 4-auto or 4-hi. It does not make it when in 4-low. The humming only happens during speeds between about 25-45mphs. Mechanic could not hear any grinding in transfer case when on lift. Is this simply a problem from having different brand of tires on front?