Popping sound in headers

I'm pretty sure the exhaust is tight. I re-did it from the collectors back myself and cannot detect any leaks at the header flange gaskets. It has a manual choke which I normally release completely 5-10 secs after starting. The motor was allegedly rebuilt sometime shortly before I bought it. That is why I mentioned "unknown cam," because I was wondering if this condition might indicate the presence of a non-stock cam of some type which might exhibit this behavior "normally," so to speak.