with the engine still running (i assume it still runs),
don't rev it up, pull the #1 spark plug wire out (loosen it b4 you start the engine, leave it all the way down in the hole) using a rubber handled pliers or something (so you don't zap urself with the ignition spark <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/butbut.gif" border="0" valign="absmiddle" alt=':but:'>  
.
IF the "banging" sound gets quieter, could be bad news......could be a wasted conn rod bearing or wrist pin bushing (on piston). i jumped to this diagnosis cuz ur car is turbo'd, right?
if it's more of a "slapping" noise, i suppose it could be the timing chain prob that these cars have... <img src="http://www.zilvia.net/f/iB_html/non-cgi/emoticons/huh.gif" border="0" valign="absmiddle" alt=':huh:'>. good luck!