center diff broke or rear diff?

[mdkenley]

first off thanks to everyone that helped me with my spur gear problem it was the mesh and the paper trick worked great.now for the new problem i have a loud clicking at takeoff. i want to know how to tell if its the center diff or the rear? when i spin the rear wheel the wheel turns the same direction unlike the front wheels the noise seems to come from the trans?both front and rear diffs are stock.my set up is mm/9xl/3s80000mah truerc lipo if this helps?my first race will be this Saturday would like not to tear the whole transmission apart if not necessary. Any help will be greatly appreciated. Thanks.