I had the same on my ST220 and boy did it like to play tunes..
it started when it was left running in the MOT bay superb al round the bay it echo'd
gave the idle control valve a good clean out the stuff coming out was filthy
ever since been fine..
only seen the pipes on the Mk2 mondeos and cougars - some were fitted adn some werent
touch wood mu current 200 is behaving at the moment
sods law says...