Hi all. My 2007 w/3.6 threw a p0420 (bank 1) and I hear rattling in the resonator.

Should I replace both cats or just the passenger side (bank 1 from my research, correct?)

I read that GM extended the warranty to 200,000 miles, but that's only for V8 4.6, right?

Should I buy something from Auto Value or Autozone? OEM or Mangaflow? All I want to do is change the converter with as little problem as possible.