ok I installed a header on my 89 hatch with a b16a 1st gen swap, everything sounds good until i shift or let off the gas, when i do i get a small but noticeable crackling and small backfire, ive noticed this in all civics, integras that have aftermarket headers, but what actually causes this?

mine does it...like a little pop....i don't pay it any mind...but not sure what causes it

Make sure there's no leak on your header gasket and tighten all bolts properly... Popping and backfire usually caused by leaky exhaust.