About a month ago I bought an '03 4X4 SCrew XLT FX4 with 3.73 LS. I've noticed in the past couple weeks that the rear end is making a weird kind of rubbing noise (that's the best way I can describe it anyway). I only notice it at speeds below 20mph while turning. I just changed the fluid, used Lucas 80W90 because it was the only brand I could find locally that any place had more than one bottle of. I know the LS rears need the friction modifier, but Lucas claims it's not needed with their oil. It seems like it really only does it when I'm pulling into my driveway in the afternoon. I don't really hear it pulling into work in the morning. I'm guessing that's because it's cooler in the morning and helps to keep the fluid a little cooler. I know the rear ends are known for being a bit noisy, but I'm afraid there's something wrong. Anyone have any ideas?

One other thing, should the rear get 75W90 or 75W140? The tag on the diff says 90, but the manual says 140.

 

LS rear ends make a clunking noise or grabbing noise when you turn since the wheels are locking up and are going different speeds. One tire will drag. If this is what you are hearing, it is normal.

 

75W140 synthetic and a bottle of friction modifier from Ford.

 

yeah mine had the same problem when i bought mine with 39,000 miles and just changed rear fluid and no more problems

 

I'm honestly not sure what I'm hearing. I think it's more of a gear noise, but it may be that the tire is dragging and causing it. It's kind of hard to tell. Regardless I haven't heard it as much the past couple of days. I think I'm going to put on a different cover that holds a little more fluid and has a drain plug so I'll change the fluid again and put in 140. Now to make a decision as to which cover...