2005 f150 super cab, 5.4 liter, 4x4 107k miles

I posted this in a different location but no response so I'll try again.
I have a 12 bolt rear differential and the tag on the door has an axle code of "B6".
No noise under power or braking but if your driving along flat road and not much power is needed to keep the truck moving then there is a hum almost sounds like faint grinding but not quite. The differential housing gets hot to touch after a 30 mile drive down the interstate.

My brother was the previous owner and this has made the noise for years so it's not critical but annoying. I removed the dif. cover to change the oil. All the teeth are smooth, no rough or grinding surfaces. No play at the wheel end so I don't believe its a bad bearing. Oil was used but not burnt up or sludgy and very few metal flakes at bottom of housing. Thought it looked good for the age. Also, both wheels off ground and truck in neutral. There seems to be a lot of resistance while turning the wheel. I don't think the brakes are dragging but will eliminate that possibility this weekend.

Someone please tell me what rear axle I have. Do you have any suggestions about why its making the noise? How to repair it? is this a posi dif. ?

Thanks!

 

Another note: which is part of the, is it a posi rear end question. While both tire are off the ground and I spin one by hand the other spins the same direction. Is this normal for the axle? My Ranger; spin left tire forward and the right goes in reverse. Thats the only thing I can compare it to.

 

More research has given some answers. My rear axle is a limited slip 3:37 ratio with a 9.75 ring gear.

 

When you took the diff cover off to change oil, did you note the wear pattern on the gear teeth face?
The center of the tooth on the ring gear should be shiny, meaning that's where it and the pinion gear mesh. If the wear pattern was off in any direction (front of tooth, back of tooth, leading edge or bottom of tooth) that would indicate the gears need to be shimmed back into proper alignment.

My 182k f250 has a whine from the rear axle, and it's differential gets warm too. Unless it's extremely loud or grinding, Ford Differentials don't readily break.

 

thanks. I will try the additive since I just replaced the fluid & gasket. I assume I just suck out "X" ounces of gear oil then replace with "X" ounces of additive. Is this correct?

 

You can, but Its such a small amount compared to the gear oil you (probably) won't go over full.

I kinda doubt it will help the noise, but it won't hurt.

Try it and let us know!