What you're hearing is the retarded amount of slack ford sets into the rear end at the factory. It may be "normal" because most mustangs have it, but it is also retarded. It can make the car sound like there's something wrong with it, and I believe it can lead to drive line problems in the long run.
Luckily, there is a way to correct the idiotic ford settings: go get yourself a set of 3.73's or 4.10's. Have a reputable speed shop install them, and make sure you tell them to set the rear end at the tightest possible setting because you don't want anymore slop in the drive shaft (it's called backlash I think). Pretty sure the setting to ask for is 0.08. Now your car won't sound like the drive line is about fall off, and you got yourself a steeper set of gears in the process.