Loud is relative. What might be loud for you might not be he same for me.
I had gear noise problems in previous cars (not Mustangs), and basically, if the whine is louder than the outside air rushing by your car at 60 MPH or faster, the rear gear is chewing up its teeth. It's time to rebuild the rear end.
If this is the case, take it in now to fix because it will only get worse and become more expensive to repair. If left to the point that your car begins to vibrate, a complete rear end rebuild is needed and that will cost you over $1000 to fix at a shop. The bearings by that time are shot and have wreaked havoc to everything in the diff housing and your axle shafts.
If your car is still under warranty, take it in immediately to have a look and repair if necessary.
Squirrel of Death!
Do not **** off the SoD!!!!