my 2008 a3 2.0t 6mt is making a metallic rattling noise. is there anyone out there who might be able to help? many thanks in advance. here are the details:

the car was bought new. it has ~5k on it, suburban traffic with mild-aggressive weekend driving (avg ~23.5 mpg, long drives low 30's). some WOT, but i was reasonable when breaking in for the first 3k. there's nothing extremely wrong with the way the car drives - i love it. handles fine. lots of power all over. the sound has no clear effect on performance. i'm running stock.

WHAT: recently i've noticed an intermittent metallic rattling/scraping noise. the sound is soft - i have to turn off the radio to hear it and other cars'sounds will drown it out. it's maybe like keys on a keychain but not as ping-like - more metal rustling/scraping. possibly like the infamous BB's in a can or little stones or something scraping/rattling around somewhere.

WHEN: it only happens when i dip gently into the throttle. i can most easily cause the sound by holding rpm at 1800 in neutral and dipping into the throttle to raise slowly to 2000-2200. i can also do it when rpm's are falling and i dip into the throttle to cushion the fall. clutch position doesn't change the sound. the car must be driven for 30-45 minutes for the sound to be possible. the sound happens in a range of weather conditions, though the car lives in the sf bay area.

WHERE: unknown. on two dealer visits, different mechanics localized the sound to under the hood. one said it could be a differential gear since the sound was loudest at one wheel well (i forget which). another said the problem was predetonation/knocking because he found that the sound got worse when he used fuel additive. however, i'm not sure this mechanic actually heard the noise i was describing - because in my opinion the noise wasn't worse after additive cleaned out the carbon.

MISC: fuel lines have been secured as per the service bulletin for 06-08 a3's. my dealer tweaked heat shields to no effect. they have also localized the noise to the "engine" with a "stethoscope." i think the noise began after the first check-up at 2.5k - got castrol syntec 5w40 and tire rotation. oil level is now stable near maximum. once before the check-up, some gravel got stuck somewhere (brake shield?) and caused metallic scraping/clanking (while moving) in the back of the car. resolved on its own. i crawled under the car and found no obvious gashes/dangling hardware.

currently, the dealer is offering to do a long block replacement (SCARY) based on the detonation/knocking hypothesis. though it's under warranty, to me this seems to be an extreme, last-ditch effort. i don't think i have detonation/knocking. i'd rather not get a new long block if i don't have to. what should i try first?

i've read the other forum threads about noises, but in the interests of soliciting the most unbiased readings of my situation, i'd prefer to avoid mentioning any specific possibilities of what the noise might be. would any of you be able to offer your advice?

thanks again.

p.s. anyone in the sf bay area know of a good audi mechanic? i'll be sticking with the dealer until my warranty runs out, but after that...