I guess we were right. Dave and I had this one on our minds from when we first thought about it. Since you are the second person to report that it is being changed under warranty, maybe there is some more hope for others that are getting the issue.
That's what I'm thinking. It seems to be coming from the upper part of the block. It's similar to the sound of that intake control valve closing, but more metallic. It just has me stumped.
Maybe it really is the VVT-i mechanism. When you see a video animation of it working, you can kind of picture how it can generate a knock/rattle sound if it doesn't have appropriate oil pressure in it.
The mechanism is like this for those that don't know:
Having a quick look at the service manual, maybe it ties in with the so called 'lock pin':