Over the last few weeks I have noticed a noise on my 04 FX4. It starts at around 1400rpm and goes to around 1800rpm on mild acceleration, more loud even under load like on a hill. It doesn't sound like a rear end howl at all. its hard to describe but sounds almost like rollerskate wheels on cement or maybe a shopping cart rolling real fast on pavement. It doesn't do it on coasting or deceleration at all. Only under a mild load. I checked all the heat shields, took all the pebbles off the skid pans that were wedged in and can not seem to locate it. I had someone even hang out the window and listen, they stated that it seems to be coming from the front part of the truck. Anyone else have any ideas of what to check. its not motor related it seems and will do it in 3rd and overdrive mostly again when the load is the most on stuff. it does not do it in park if you hold the rpm in that range either. It doesnt do it all the time but is getting worst it seems.