increases with speed, not RPMs. It's on my 2000 Ford Excursion 4x4 6.8l v10. Within the last year I changed the locking hubs and hub assembly and the tie rods, sway bar links, ball joints, stabilizer shocks, etc. A mechanic changed the serpentine belt and the entire AC system. When we had the snow storm a few weeks ago my wife said the sound went away when it was in 4x4. Couldn't hear it at all even at freeway speed. Turn the 4x4 off, and the noise is back. I just pulled one of the front tires cause I thought it might be the locking hubs because they got dirty when I was changing the ball joints. That seemed fine. When it was lifted up I spun the wheel and it's a little stiff and it feels like the brakes might be kinda tight. This is the first vehicle I've ever really worked on so I really don't know much about cars, just figuring it out as I go. Any ideas what the noise could be from that generic description? Anything else I should check before I take it to a mechanic to check out?