I have a 2007 mustang that makes a rattling type noise whenever I drive 55 mph. If I drive below or above 55 mph, my mustang does not make that noise. It sounds like there is a plate that is loose, but when you open the hood, nothing appears loose. The car doesn't shake, it just makes that noise at 55 mph. The sound seems to come from behind the A/C unit. I'm not sure what to do about this. Any advice?