Ive had my greddy evo2 catback on my sedan for about half a year now and there is a certain sound that keeps getting louder and lasts longer. Its really hard to explain the sound but in low rpms and when the cars not completely warmed up it makes a sound like a piece of metal is scraping the ground. If i punch it, no sound, but if i go semi slow it makes the sound for a few seconds then stops then will start again. If anyone has any ideas please let me know, the only thing I could think of would be the water pump

 

First thing I would do is look under the car and make sure nothing is rubbing underneath.

 

Could be a heat shield rattling?

Why would you think water pump if you think its related to exhaust??

 

I could be wrong but I thought any engine related noise could be heard through the exhaust because when the cars moving the noises can travel to weird places. I looked under the car and everythings tightened down, no loose straps or hangers or anything scraping. And if it was something scraping why would it only do it in low rpms?