Another weird noise

[Tem]

Ignoring the normal all-around-rattles, anyone have an idea about this one? Kinda sounds like the clattering I got when the engine blew, periodic and related to revs, but somehow this seems muffled/plastic...

Also, I only get it during the 1st 1000->2500rpm acceleration I do after starting the car on the morning. I don't get it when I reverse out from my parking space, only on 1st. And I don't get it when I start the car during lunch break or after workday, only on morning (a bit colder then). When I change to 2nd it goes away and doesn't come back before next morning. Annoying as hell and starting to make me worried...but I don't even have a good guess  s:oops: :oops: s:oops: