I just started noticing this on my 05 @ 30k too. Sounds like a metallic chain, almost gearing type sound, hard to explain. Only hear on acceleration - especially when you're going slow and your engine is at a higher than usual RPM - for instance in 2nd or 3rd with RPM's around 3k - 4k..

One thing to note though - I only started hearing this after 30k - which, incidentally is when I switched over to synthetic oil.. Coincidence? Possible cause?