ok today i put a aftermarket header on my d15. then after i started driving i noticed a little rattle or vibrating sound coming from under the car. i looked and the header is tight. but then i noticed that there is a cover over all the stock piping like a shield or something. could that what be whats rattling??

 

Yeah it could be the heat shield around the exhaust. Also check the clearance between the header and any surrounding parts.

 

check to see if you tightened the bolts to spec and that you have the donut gasket that connects the header to the cat.