Ford: Low vibration/humming, ball joints, rear tires

Question
Hi Tim,
I have a 2005 F150 FX4 5.4L with about 115,000km.  I've recently started getting a low vibration and hum when I'm driving.  It's very intermittent.  It sounds like it's coming from the front of the truck but I could be wrong.  What I have determined is that when I engage 4x4 the noise and vibration goes away.  I have had the front ball joints replace, new tires installed and balanced and the seal between the transmission and transfer case replaced recently as well.  This vibration was happening prior to this work so I would be surprised if they are related.

Thanks for you help,
Tim

Answer
Possibly a driveshaft center bearing (if applicable) or a universal joint?  check the joints with e-brake on (parked on flat) and truck in neutral. look for play or dusty rust around the cups.. some movement at the center bearing is acceptable as it is rubber mounted.

If nothing found, then jack front and rear tires off the ground on jack stands. Start truck and drive through speed to see if noise reappears. If it does, then you would need to carefully listen to various areas to see where noise is loudest.

I suspect a driveshaft or differential problem. In 4x4, the front diff and rear diff changechange the load distribution.