2005 Mustang GT

Hello, I got a problem with my 2005 Mustang GT. When I shift from 2nd to third, or 3rd to fourth, or when I do a quick acceleration (in the 3rd or 4th gear), I hear a sound that sounds like a rapid tapping or some type of metal resonating for about a second. I took it to the dealer and they told me that it is a "normal" characteristic of the transmission. I am very skeptical because it sounds nasty, and I am pretty sure that Ford engineers would not settle for such a "harsh" sound. Has anyone else experienced this? Any ideas? Feedback would greatly be appeciated. Thanks.
-Abzuli99