I had the same slapping noise coming from the drivers side. It was loud as hell at times the completly quite at others. So i threw some seafoam throught the oil let it run then changed my oil to10w30 . Still didnt fix it even though my car ran a little better!

To continue this story im out driving tonight and hearing that noise thinking i got a lifter slapping like all hell. Anyway I got on it a lil bit and heard a loud bang and a blowing air noise. Got it home and evidently when my tune up was done they didnt get the spark plug good and tight because the damn thing was hanging by the wire. So you might have had the same problems and if when you did your header install you tightened your plugs you probably fixed it then. So i will get up in the morning and make sure i didnt lose any threads in my block of my motor and make sure that was my only problem!