just installed a dc sports 4-1 header on my 2000 civic ex, now i get a REALLY BAD rattle. has anyone experienced this and does anyone know how to fix it? it seems like where the 4 tubes come together, underneath the transmission i believe, its kinda close could that be it?

 

There are some metal brackets that held up the original pipe in front of the cat. They're bolted to the engine block. You probaby just need to get under there, check clearances between the pipes and their surroundings, and also probably remove those metal hangers.

 

i removed that hanger your talking about. like i said, where the 4 tubes come together, underneath the transmission i think, its really close, coudl that be it? i wouldnt imadgine it would be tho seeing as how everyone who has one would have that problem. however, i didnt think of this at first, but where the header connects to the block, the 4 bolts on top and 5 on bottom, the top middle right nut snapped, so there is not nut there. but there is 2 to the left, 1 to the right, and all on the bottom. could that be it? it seems super tight tho

 

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Don't think the bolt would matter. If it caused anything it would be an exhaust leak I think.
Still fix the bolt though.

Apart from that I'm not sure.

 

Exhaust leaks can sound like rattles. You need to get the car in the air, start it, and get underneath it*. You'll find the problem if it rattles all the time.

Just make sure there's a jackstand beneath each raised wheel... and not a jack. Don't get under a car that is supported only by a jack.