2000 Explorer front end hum

I have 55000 miles on Explorer 4*4 with auto trans and notice a hum that starts at 30 mph as I slowly accelerate to 55 mph. I feel a slight vibration in the seat also. The tires,ball joints and frt end alignment have just been done. I noticed that when the vehicle was jacked up and I put the trans in drive the front wheels started turning . Is that correct ? Any ideas on the front end hum and what to check ?

if you have auto 4x4 vs. 2wheel,the wheels should turn. does the noise change while turning left to right? wheel bearings. change the fluid in the tranfer case for the vibration.