There is some resonance that comes from the single walled construction of aftermarket mufflers, thats' why factory used double walled w/ shape reinforced/stiffened mufflers, take a look at a 997 muffler for an example, wrapping the outside can lead to moisture condensation and accelerated corrosion
There are several best practices for the muffler internals one is is to dump the gases into a mixing chamber like the this GT3 w/ an acoustic x=over between the banks
and this BMW which is mostly flow separated but has an acoustic link between the banks
another is to keep the banks totally separated like this 993
A third is to use dead ended resonators
