2005 Ford Mustang Noisy Car in the Col

My 2005 Ford Mustang makes a loud, irregular clattery noise in fifth gear and in neutral when the car is in motion. It is a manual transmission. This only happens when both the engine and the surrounding temperature is low (45 degrees F or less), so it's been very hard to have any mechanic diagnose. Half the time, they don't even believe that there's a problem with the car simply because the conditions have to be so precise to hear the very, loud and obvious noise when it is cold.

I have the car in the dealership now (for the third time) and they still haven't had any success in figuring it out.

Any help would be hugely appreciated.