I got a used stainless steel header and installed it on my b18b1 and there is a metallic noise coming ONLY FROM 2500-3200 RPMS and any load. It is a bit bent on the downpipe portion.??

 

sounds like somethings not tight.

 

its probably something rattling because that is the rpm where rattling of heat sheilds always occurs. check the bolts that connect the downpipe part and make sure the header isnt hitting the oil pan or anything else underneath.

 

I forgot to add that it is a SS 4-2-1 DC Header, bent a bit a bit after the 4-2 section.