I have a '12 GT/CS and I had to take it to the shop for a creak in the front-end just 3 days after I took delivery. Turns out it was a bad front lower control arm bushing. They replaced the bushing and it rides like a dream again with no creak. Must have had a bad bushing from the factory. It started the day after I bought the car, but by the time I got out of the dealer and home it was 11PM, so I had to wait to take it in. Plus I drove 3 hrs away to get it, so this time I just took it to the local ford dealer. I had them re-torque the strut mounts, strut tower brace, and sub-frame too because they moved on me. I have a fairly steep driveway and have to take it at an angle, guess it wasn't torqued right from the factory so they slid. Driveway isn't incredibly steep, its just the way the road dips down to my driveway before inclining that screws me. I built a ramp so all is good now, but figured I would share. My creak was from the front and mostly noticeable at slow speeds when I hit a bump, which would cause a creak/clunk that could be felt in the floorboards.
Good luck on the fix! Use that warranty while you have it, these cars drive amazing and it's easy to find yourself just wanting to go for a slow cruise ALL THE TIME and rack up those miles =)
HIS: "Night-Mare": 2012 Mustang GT/CS, Black on Black, MT, 3.73 Gears, HID's, Security/Comfort/Electronics Packages. #2 of ????
HERS: "Blue Steel": 2012 Mustang V6, Kona Blue, AT.