Minor whining noise from rear?

[hwydave]

Happy New Year everyone!

My '09 Explorer V6 starts to developed this minor whining noise when come to a sudden stop when climbing up hill. I can hear a small "thump" noise (like it is slam into gear) and then a 2 second or so of whining noise. It only happen when go up hill, not down hill. There is no noise if I curising to a stop when going up hill. This noise situation happened after I drain and fill the rear differential about 20k miles ago (use Motorcraft rear fluid) - truck has 102k miles on it now. FWIW, when I drain the rear differential fluid, there were some dark gray powdery substance in the fluid (graphite additive?). I got the truck at 26K miles (still under warranty at that time) and I doubt the previous owner add any aftermarket stuff and I never add any aftermarket stuff.

Does anyone come across this kind of situation? The truck has no limit slip. Will the friction modifier help the noise situation? Thanks for the help!

 

[digifoss]

The thump would make me want to check the U-joints but It's funny that it started right after you changed the rear fluid. The dark gray powdery substance is probably metal specs and flakes from wear. Check to make sure the fluid level is good. Change the fluid again and see what comes out this time.

 

[hwydave]

Thanks, Digifoss, for the suggestion. I do plan to change the fluid again this weekend while doing the oil change. One more detail, WFIW, after the last rear axle fluid drain, I was hauling about 300 lbs of gardening stones in the back (rear seat folded down and eveningly distributed the weight around the back). That was the first time the thump and whining noise started.

 

[hwydave]

So I replace the rear differential fluid with Motorcraft fluid today. The old fluid was in there for about 10k miles and it come out nice and clean. I replace them anyway. I check the suspension and U-Joints (front and back). The joints are in good condition, no rattle. The clunk and whining noise on up hill quick stop still there.

Any suggestion what might be the issue?