exhaust drone/harmonic resonance

Glad I saw this thread. I installed the Summitt kit with their mufflers several years ago. Not a bad sounding system. Last year I installed an X pipe and it made my car quiet down. I was under it the other day and noticed one of the mufflers has rusted out so I'm looking at mufflers also. 340 warmed up, Hooker Super Comps, 2.5" pipes all the way to the bumper and mufflers installed in the stock location.