My best guess is that it is some weird harmonic from the exhaust. Maybe due to the fact that 3 & 4 exhaust are connected at the manifold due to the divider being poorly installed by PER, or maybe this is just the way headers sound - I wouldn't know since I've never had headers before. You don't really hear the throbbing sounds out the tailpipe, but maybe the muffler removes it by then.
Headers do sound different due to the thinner metal compared to a cast iron exhaust man. They can have a pulse sound or tick usually you can just hardly hear it sanding (hood shut) next to car or sitting inside at idle or low speed.
Any thoughts on that one? My guess is that maybe I have a rocker adjusted a bit too loose (I have been a bit paranoid about overtightening them). I can't get the sound to occur when the car is stopped, so I can't put my ear to the valve cover and try to hear which one. Should I just go through and tighten them all maybe one quarter turn more? Again I'm paranoid of breaking something, and I don't want to bend any rods again...
I always set them engine warmed up and running (at idle) works perfectly the only draw back is sometimes there's a little oil in spots to clean off.