I will get a faint rattle if I rev the engine with the gearshift in neutral and the clutch pedal up. The rattle is most pronounced just as the revs change from increasing to decreasing. It's not loud at all, though. The rattle sound is not there when the clutch pedal is fully depressed.
Had the car at the dealer for something else and asked about this - my clutch/tranny rattle sound was diagnosed as 'normal.' Others on the 335 boards have complained that the clutch in this engine/tranny is just noisy. My two prior Audi S4s didn't do this, though the Mustangs I had before that did. (No flames please, just observing!
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I don't hear the specific noise that the OP describes - which I gather is happening as he is moving away from a stop, or even when letting up the clutch pedal w/ the gear lever in neutral.
It may be a normal sound, but it can't hurt to have a dealer listen to it.