Rough idle when at a stop light?

Hi everyone I have a 02 explorer with
a 4.0 v6. It idles rough not really bad
but enough to hear it. Whenever I'm
at a drive thru or at a stop light when
the brakes are engaged. I have a
engine light on for 02 sensor and i
hear a sound Which sounds like a
pulley on the passenger side. Well I
changed altenator, and belt tension
in the last 6 months and the only
other pulley on that side is the idler
pulley. Would it be both? I'm
changing my 02 sensors and pulley
anyway