Popping out of exhaust, 2GV

Good afternoon,
I have a 1970 Buick Skylark, 350 W/ 2bbl Rochester 2GV. The engine has been refreshed,with approx 5,000miles on the engine. Before the refresh, I had a slight popping(thup-thup) out of the tailpipe( duals w/iron manifolds) it is timed correctly, with no issues on vacuum. Have any of you had this issue? It goes away at 950-1000 RPM, if idle speed is raised. Slight cam profile, but otherwise stock. It still does this,before and after. Anyone else have this issue?
Thank you!
 
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To narrow it down, with duals, you must know which bank is making the popping noise. Then, with the engine off, disconnect one of the 4 plugs on that side and keep going until you determine which cyl is causing the issue. This will help to narrow down the problem. Did you replace the lifters or pushrods when you refreshed the engine?

Gary
 
What kind of mufflers?
A pair (bad pair) of Walker 17749's were spewing their fiberglass packing and it sounded like popcorn.
 
Just because you mention a slight increase in idle eliminates it, how about advancing the timing 4-5 degrees and seeing if that eliminates it too?
 
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