i have this noise in my 06 WK. you're right...it happens around 12 mph and only once. i assumed it had to do with speed sensitive door locks. mine only does this upon start up and never after. so it didn't really make that big of a difference to me. plus, i had never had auto door locks that were speed sensitive so i just assume this was normal.
i have also read on here somewhere that it had to do with the driveline removing the extra slack. (i know that is not really what i read, i just can't find the thread nor remember the exact wording of how it was explained.) i think this is common in the WJ as well.
2006 Jeep Grand Cherokee Laredo (WK) 28E
4.7 V8, Quadra-Trac II, Black Exterior, Light Khaki Interior
Upgraded: Mopar Sport Utility Crossbars
2008 Jeep Patriot Sporkt (MK) 26E
2.4L, CVVTII, 2WD, Black Exterior, Dark Gray Interior