Accelerating Noise.

I have a 1995 Ford Explorer with 169,000 miles on it. Everytime I accelerate it makes a humming noise that sounds like it's coming from underneath the front of the truck which seems to come on around 10 MPH. It's not a whining noise, it just seems to get louder whenever I'm stepping on the gas. It seems to be running smooth to me, it doesn't stutter. It's not loud when the car is just sitting still only when accelerating. I just recently had a tune-up and am hoping that it is just the exhaust needing to be replaced. I asked a friend what he thought and he said he didn't think it was the exhaust because it's not loud on the outside when the car is sitting still. I was wondering what the noise could be?