jeep11

jeep is making a humming sound that is at 40mph and at 65+ it is a humming noise that will go for like 5 sec then soften then come back then repeats until i go under 65mph. wheel hubs r new, brakes n rotors, also took it in today and had all the wheels balanced.. ANd a soft grinding noise just before coming to a stop.???? i put it into 4x4 and it seens like it grinds as well. please help if you have any info. took it in today and the mechanic said he couldnt find anything wrong. could this be a u joint problem? not sure if they could be related. it is a 1999 cherokee sport 4 door 4x4 4.0L. THANKS!!