"Rattling" sound in '97 Honda Civic...

Hello, I just had the pipe directly connected to my muffler (I think it
is sometimes referred to as either the "B" pipe or "C" pipe) replaced
on my '97 Honda Civic DX due to rusting. It was suggested I have my
muffler replaced also, but I didn't see a problem with the muffler so I
just had the pipe replaced instead. After having the car fixed, I
noticed a rattling sound coming from under my car... I usually only
hear it when I drive at very low speeds or apply the brakes (mostly
when I'm at a stop sign or red traffic light). When I am driving at a
"regular" speed I don't hear the rattling sound at all. I've read that
this may possibly be some heatshield problem or maybe even at problem
with the cat converter?? I only noticed this after getting my exhaust
pipe replaced at Midas last week (never heard the sound before) so I am
wondering if I would have to perhaps get a new heatshield? Does anyone
have an estimate as to how much it costs to get that replaced??
Thanks!!!

Julie