Is the CAT still intact? The matrix can break and rotate in the can causing back pressure and other fun. A knock usually sounds loud and harsh, ball peen hammer on concrete .. Ping is lighter and more broad band across the RPM range. Excess back-pressure can create excess pressure in crankcase as well... Dave. __________________ Dave - PDX 1995 318ti - Active Black and Tan. 2005 330xi - Mtech 1 - 6spd - Orient Blue/Black |