Noisy Rear End

[colnago]

All,

I've been setting up to rebuild the rear hubs in my F250. A couple of weeks ago, I replaced the rear differential fluid. I used Mobil 1 75W90, which included friction modifiers for a limited-slip. I bought a bottle of the MotorCraft friction modifier, but since the Mobil 1 already had some, I didn't use the MotorCraft. Now I have two new sounds coming from somewhere in the rear: a chirping sound when I'm going ~20MPH (which stops if I either give it more throttle, or back off on the throttle), and a clunking sound that speeds up as I accelerate, then goes away once I get to ~10MPH.

I _thought_ the clunking sound was something in one of the hubs (probably on the driver's side), since it's not muffled. I'm thinking it is probably a broken spring for one of the brake shoes, but I'm not having any driving, grinding, rubbing, or dragging problems (it's just a metallic "clunk" that speeds up with the truck, then stops). There is nothing external that I can see that would be causing the clunk. The chirping could really be anything, but my first thought was U-joint or driveshaft support bearing.

Could either of these be coming from the rear differential instead? Even though the Mobil 1 had friction modifiers, should I go ahead and add the MotorCraft? Can a limited-slip differential have too much friction modifier? Should I drain the Mobil 1 and use something else?

Any pointers appreciated.

Joseph

[Busboy]

Rear end noise happens to all of us occasionally.