'03 I35 rattle from under the hood

[skassamali]

I have a rattle (it's been called valve clatter) that occurs while I'm in drive and driving down the road and occurs just under the 2000 RPM mark. It doesn't happen if I rev the engine to same mark while in park/neutral, or even if I raise the car on stands, put it in drive, and rev to 2000 RPMs.

I've run 4/5 cans of BG 44k and done a fuel injection service, thinking it was fuel related, but no luck. Dealer thought it was air-flow related so they replaced the air flow sensor and reset the ECU about a week ago, but no luck. They said something about the alpha readings not being what they're supposed to be - but not sure what that means.

Any other ideas as to what it could be?