Popping sound in headers

It is a 4sp manual. Suppose I'll get the timimg light out and check the plugs just to meake sure everything looks good on the ignition side. It seems to run really well, though. Pulls strong all the way up to 6k rpm. That is the only reason I have not checked any of that stuff yet. I did pull one plug as soon as I got it home the first time and it looked fine.