Chevrolet: 2000 Malibu ?'s

First, and most importantly, what is the code P0420? My check engine
light came on, I had it read, they gave me this code and suggested
that it may be my catalytic converter. I replaced that last year...
could it be the o2 sensors? If so, is that an easy fix for me to take
care of? Where are they located? My car failed its emissions test
and I need to repair this as soon as possible...

Secondly, there is a distinct "tick-tick-tick" noise coming from my
front wheels (it speeds up and slows down according to my speed). It
started on the right side and has gradually progressed to the left as
well. I had originally thought it was something in the hubcap, but
since it's on both sides now I doubt that's the issue. Anyone have
any similar issues?

Thanks guys,
Keith