weird noise after header installation

[Yellwaznboy]

Hey guys, I just installed my HS header a little over a week ago. After the installation, everything went well. But since this monday, whenever I drive for awhile (it's an automatic), then slow down to back the car up into a parking space or something, I hear this high pitched grinding sound. So I left the car in reverse, pulled the hand brake and popped the hood to check and see where the sound was coming from. At first it sounded like it's coming between where the header connects to the engine, but later it sounded like it was coming from underneith the car where the header connected to the second cat. I never have enough time to tell where the sound is coming from because the fan would kick on. Now if I know what I'm listening for, I swear I could hear it very faintly even when I'm driving, but the h/i/e is too loud to really be able to tell. Could there be a bolt loose or is it something else? I still havn't checked my butterfly screws, but I don't think it could be that right now. My car's built in Jan./03.