Hello, I have a 2004 F150 that I bought used with 99,000 miles on it back in Aug 2014. It now has 110,000 on it. There is a (what I always heard referred to by others) as a singing noise coming from the rear end. It seems to be more prevalent on the left side but that might be because I am driving but I can hear it better when I turn my head to the left versus to the right. I hear this noise when I am traveling roughly 40MPH and above. It starts out a little lower tone then gets to a slightly higher and much louder tone when I get to around 50MPH and above. It seems to be only when I am accelerating, and when I let off the accelerator it goes away until I start again. It doesn't seem to change very much uphill or down hill, or turning left or right. Also it seems to be more noticeable older roads as freshly paved roads don't seem to be as noticeable. on It seemed to me the that issue was wear of the rear differential gears. By the way this is not a limited slip rear (unfortunately). I have read on some forums that Ford put a 75-90 gear oil in their rears and that it should actually be a 75-140 so I have done that hoping that might help fix the issue, it seemed to help for a couple days but then it showed back up so maybe it was just wishful thinking. I am trying to figure out how to get this to go away hopefully without having to replace the gear set. Anyone have any ideas of what might help this. My next step was to rotate the tires and see if that helps but I haven't gotten there yet. Maybe this weekend. Thanks in advance.

 

You could pull the plug on the rear diff and check the oil. If the gears or bearings are failing the oil will have a silver color.

 

I just changed the gear oil roughly 500 miles ago when I put in the 75-140 oil. The oil that came out was black and didn't show any real signs or smells of burning or having any unusual metal in it. There was a very limited amount of stuff on round the magnet on the cover which seems normal to the other rears I have seen. Thanks for the thought.

 

It sounds to me like the ring gear/pinion gear noise. Perhaps someone "rebuilt" it and didn't set up the gears properly