If it sounds like a lawn mower its because you don't have a good seal and exhaust is leaking. If you get under the car while its running you will be able to feel where your leak is.. if you are sure its the header/downpipe connection, they should be connected with spring bolts .. Just tighten them up again.. Sometimes heat and vibration will cause them to come loose
ok i tried to tighten them and it didnt work. so i went and bought a new gasket for it. put the gasket in and its still doing it. so ive figured out that i have a hole in the flex pipe of my down pipe . what should i do cause this is killing my gas mileage.