I added a full bottle of Friction Modifier to the 06 Stang's rear two weeks ago and. WA-LA................... Axle moan is gone......................
Looks like that's exactly what Ford left out of the rear gear lube................. Someone undoubtably thought it would save money, but ended up causing a problem instead.

And adding a 4oz bottle of friction modifier will not over fill it. I filled mine till the oil was coming back out the hole.
i always say that backwards for w/e reason