Funny clanky sound in reverse

[Racer X-8]

Yeah, when I put it in reverse, there's a funny clanky sound, coming I think from the engine compartment. Automatic transmission. Sounds like a washtub being repeatedly hit lightly with a drumstick or something. Gets a little louder when applying some gas in reverse. Only in reverse, not the slightest hint of it in "Drive". Do we have any Yaris mechanics or somebody who knows this one?

edit: Oops, found a duplicate thread.

http://www.yarisworld.com/forums/showthread.php?t=1774

sorry... (why can't we delete new threads?)