I have had the same problem but it started at about 11k I took it to the dealer. They said when they drained the oil metal shavings came out. They told me they spoke to Ford about it and the fix is to replace the pinion gears. They did and said if it comes back bring it back in.

22k the problem is back and they are replacing everything AGAIN.

What really scares me is what is going to happen after the warranty is up?

Good luck, my dealer is a pain in the you know what to get parts replaced but they told me that this was Ford's fix for the problem. Be persistant with your dealer.