Nast;y noise

my 2000 ford focus has what seems to be a loud rattle in the exhaust system. this noise subsides after car has warmed up.

Could be a loose heat shield.

Does it come and go with changes in RPM? If so it’s almost sure to be a heat shield. I’d suggest taking it to a locally owned muffler shop, not a chain like Meineke or Midas, because all they will want to do to sell you a new catylic converter or whatever is shielded by the errant shield. A local shop will be able to simply put a tack weld on the shiled to keep it from buzzing. So COULD a chain shop, but it wouldn’t be as profitable for them, so they are unlikely to do it. JMHO from past experience.