Okay, I am hearing a strange noise from my car. It is a non-rhythmic knock type sound. It does seem to increase in frequency as I rev the engine, but does not seem to be proportional to engine revs. I cannot hear the sound when I open the hood. The sound is more pronounced when I am on the ground at either side of the car behind the front wheels, but not from under the front air dam. The sound does not go away when I press in the clutch. I have no driveability issues whatsoever. The engine revs freely and does not miss a beat on acceleration. The clutch/shifting seems just as smooth as ever. The sound does not seem to decrease as the engine warms. I have replaced both cats (DEC mfg) a few months ago, but am not ruling them out as a possibility. But the cats do not rattle when I strike them with a wrench. It is a lower toned sound than the "marbles" cat sound. Could this be the throwout bearing? Please say it ain't a main bearing or a conrod ... Please give me your advice. I have tried to supply all information I can think of. The clutch was changed out along with the flywheel at 90K miles. Hopefully I have included enough information as it may be a while before I can respond to any further questions you may have for me, but will do so by 08/31/06 1:45 PM ET USA. Thank you in advance for any direction you may be able to provide.