I own an 03 wrx, and I recently changed the plugs. I purchased the autolite double plats... I installed the plugs and after start up i have a slight pop(after fire) at about 4000 rpms... now I figured it could be the gapping seeing how the autolites have a gap of .044, So i pulled the plugs out and regapped them to about .030... Reinstalled and still had the same popping noise. Now I know I did not get my coil mixed up as I did one plug at a time so there was no possibility of mixing them up. I ended up pulling those plugs out and putting my NGK's back in... but this did not fix my problem, I now have the plugs that I have been using the past 3 years back in the engine and it is still popping off around 4000 rpm's. I am completely baffled and have never had this problem before.