Alero is humming while driving over 40km?

recently while passing another car, my car started to hum. my friend who is excellent with cars cannot figure out why , he has checked all the wheel bearings, replaced a couple. he also checked the new tires l put on, not them. he does not figure its the differential, nothing seems loose in car. the noise is constant, doesn't matter if l turn wheel left or right. but it does not make noise in reverse?