This guy in the local Mustang club I just joined has the same Flowmaster American Thunder muffler I've got on my '06 V-6. There's a rather annoying drone at certain mid-range RPM's and he says that putting counterweights on the muffler helped cut the drone his makes by 50%. He has since gone to duals so there's no way I compare between his and mine now. I did go back to the muffler shop and asked the guy who installed my muffler if counterweighting would help, and he said he never heard of it (doing so.) Said that the drone was in "the nature of the beast" and nothing can really be done about it. I thought I'd throw this out here for some opinions....