exhaust drone/harmonic resonance

I have the Summit 2 1/2" kit with their mufflers and manifolds, 69 340 727 4.10 rear, a little bump to the cam....450 .455 268/272 durarion No H or X pipe yet. Full 2 1/2 tailpipes to the rearbumper with the repop 2 1/2 tips. Very bad drone at 1800 rpm. The funny thing is that at 2300 rpm, the exhaust is so quiet that I can wisper and carry on a conversation. 3600rpm drones nearly as bad as 1800

I was gonna swap in the Dynomax super turbos, but not after reading this thread. Looks like the new dynamax mufflers are the answer.