Rotarius

Does anyone knows about this noise my FD makes; it sounds very deep, rich sound just like any other car with an aftermarket exhaust during WOT then around 4,500 RPMS the deep sound changes to a whoossssss til redline, kind like the old cop cars sounded while going fast. It makes this sound with the factory system too just less noticeable. I suspect the main catalitic to be plugged because of low dyno figures(210). Thank you for any inputs.