I held a hose up to the headers to help listen for sounds. The sound is definitely coming from below the middle header/exhaust areas. So I'll loosen and retighten the bolts later. I have some carb and cable adjustments to make today.
And even though you have heat sleeves on those plug wires, you'll want to get those OFF the headers, as they WILL burn the wires eventually. Probably sooner than you think.