pipe popping

I've had this issue since the end of summer and it's iritating the crap outta me.

When COLD, i have a popping from my left tailpipe. From around 2500 RPM up. I mostly goes away when the thing warms up. Mostly. It's annoying and embarassing when leaving cruises and such.

I've been thru everything and all I know is what is ISN'T. It isn't plugs, wires, valve adjustment...........I can't see , feel or hear an exhaust leak.
I need to get this resolved before moving on to other projects.

It's an IMM built 410 around 520 HP with a solid Comp cam, MSD ignition and Dougs headers. Holley 950.

The thing still runs....should I say HARD....so I'm stumped. I'm kinda thinking I got a sticking exhaust valve, but don't know.

Any thoughts from you guys really helps. Brian if your there please chime in!

Thanks guys!