My evo9 drones so bad and was curious if it has anything to do with my high flow cat ? Car currently has a turbo back exhaust. (Hks)

 

Which HKS system is it? They make a couple of different ones. I have the one with the carbon muffler with resonator and it is actually pretty quiet on the highway. No droning at all. I also have a 2.75" down pipe with a 200 cell HFC. If you put the stock cat back on, it will be a little quieter.