The rock sound appears to just be coincidental road noise, I agree that it may be the catalytic converter. If it looks to be the original then it makes more sense. If its clogging up then your engine cant push air out properly which throws off power production. Drive it a few minutes until the engine reaches operation temperature on the gauge, then park with the engine on, open the hood, and see if the catalytic is glowing red. If its badly clogged it may glow from the trapped heat. I know standard catalytics do so, but HX catalytics are wider in shape which may or may not have the same effect.
If you're not mechanically inclined enough to want to remove the catalytic to test it, then go to a muffler shop and ask them to do so or at least look inside the catalytic. If its clogged, either have them put in a new one or ask then to hone it out (remove the internals) and reinstall it. That'll resolve the issue but may cause temp reading issues at the 2nd O2 sensor which could lead to running richer or prompting a check-engine code. HXs are much more finicky about exhaust readings, what with the 5-wire O2 sensor.
