Humming noise in front end of 2007 Mountaineer

[tl240]

My wife and I just bought her a 2007 Mountaineer with the V6 engine. It has 30450 miles on it. I've noticed that when we are driving and reach 40 mph, a loud humming sound can be heard from the front of the vehicle. When you take your foot of the gas, it seems to go away, but returns when you press the gas again. Its not as loud from the passenger side of the vehicle, but it really loud from the driver's seat. Any ideas of what this may be?

[SMMN25]

At 40 MPH, I get a loud humming/howling sound as well. What did you find out about the cause??