How To Cure a "Rappy" Exaust?

I was going to fab up some long tips but adapters are not readily availible. I did find some 2.5" to 3.5" 11" long tips made by Hooker at Summit racing. I installed them last night and here are the results:

They did cut some of the rap out but it's still there. The rap seems to have deepend too and it's not nearly as annoying/attention getting. It is louder at idle and at WOT, but it's much more pleasant sounding. Strangely enough, it seems much quieter at cruise and mild throttle application. I think they dropped the frequency of the exhaust to a point where the human ear is less sensitive.

I would think, judging by these results, that a longer tip as was suggested earlier by swingingdart would take even more rap out. I also believe that a crossover like everyone else suggested would eliminate it entirely. I'll keep updating this as changes are made. It'll be awhile though, lol!