so i was driving around chillin , i turned off my radio and i heard what seemed to be a plastic bag under the car type of noise , i pulled over and saw nothing visually there , so i figured it flew off , then i was in 2nd gear again coming down at around 4k rpm , and i hear this awful noise , it again sounded like a plastic bag and a loud hiss/vibrationSOUND/ , i revved up the motor in the parking lot and everything sounded ok , started driving , happend again , took my friend for a ride and it was really loud same noise , but this time in first 2 and a little of 3rd , so i was like fukk it im gonna bring it home , on the way it wasnt making the noise so i gave it some gas and it isnt making the noise at all im gonna go out and see whats up in a little btw i recently gutted my cat , but everything was pretty tight with the bolts , i also took the heat shield off above the cat cuz its a bitch , if any of you guys know what this could possibly be let me know , its pretty dark out so i cant really get a good look at the other heatshields ,although the header heat shield is on tight i checked that one all i can think of is the heatshield in the back or possibly something in the tranny , wich i really hope not , but whatever , let me know what you think , thanks alot everyone

 

I had a similar noise when the inside of my muffler was rotting out. Pieces of the interior of the muffler were moving around from the exhaust flow and vibrating at idle. When the pieces would lodge them selves into a position that would block the exhaust flow, I would hear a hissing sound from the exhaust gasses trying to escape with increased backpressure. Maybe something similar is happening to your cat from you gutting it?

 

i got lazy today and diddnt drive , i appreciate the response tho im gonna check tomorro

 

I'd go with Mythias' hypothesis.