I have a 2000 Mustang GT 5 speed. It has longs tubes and an O/R H-pipe. When RPMs are falling between 1500-1000 there is a loud rattle coming from under the car somewhere from the front wheel well to the seats. It sounds as if I'm taking a hammer and beating two pieces of metal rapidly. As soon as the car returns to idle the noise stops immediately. I can get it to make the sound when parked and revving, but it seems to be louder when the car is rolling slow or coming to a stop like a stop sign and such. While moving, it does not matter if I'm turning, in gear or in neutral, clutch in or out, but when I'm slowing to a stop or even a slow roll and the RPMs are between 1500-1000 I hear it. Once i stop i still hear it until the RPMs return to idle. I've had it on stands and tighented all exhaust bolts, headers bolts and shook everything I can get my hands on but still can't figure it out. If anyone else has had this issue, your help would be greatly appreciated.