I have a 2003 Mustang V6, 5 speed with only 53,730 miles on the car. Last Saturday while trying to make a left turn from a neighborhood street, I gunned the car up to around 5,600 RPM to try and beat the traffic as this was hard making a left with the heavy traffic. I pulled up to my friends house as he was waiting outside and he told me my car was making a rattling noise from underneath. I did not hear this noise and thought nothing of it until tonight when I got out of school. Sure enough, about 5-6 seconds after starting the car, there was this rattling noise that lasted about 2 seconds, then went away while the car was in neutral. What could this rattling possibly be? I'm going to guess either a catalytic converter (or it's heat shield is loose) or possibly a u-joint. I hope I didn't do severe damage to anything. I'm going to look under the car tomorrow and if I can't find anything, I might just have to take it up to the stealer-ship