Lexus GX 2010+ :: 2014 - Nasty Rattling Near Idle / When Cold

So this is the second winter I've had my GX (a 2014 purchased in Nov. 2013).

With radio and other accessories turned off, there is a rattling/grating sound that is quite pronounced at/near idle, especially when cold. Sounds like a rapid PRAPP-PRAPP-PRAPP. This occurs when the vehicle is in Drive, but stopped. Occurs most often at within the first 5 minutes of operation....i.e. leaving the house or office, driving to the end of the street, hearing it at a stoplight or stopsign.

I've posted videos/recordings of it previously, which I have also shown to the dealer. The dealer said they could sort of reproduce, but could not diagnose it. I have tried to crane myself outside of the GX to listen to it, but from outside nothing seems to be amiss.

At first I was worried that it was engine knock/the car eating itself, but if that noise was representative of that, I think the car would be blowing smoke or dead by now. I have not noticed smoke, adverse effect on fuel economy or the noise at speed/under load, as you would expect if the engine were truly knocking. So I'm thinking it may be a fan, some type of underhood accessory, etc. that is the culprit. Sounds almost like a plastic component (fan, pulley, etc.) could be off kilter?? Something that more or less goes away after the engine has fully warmed up and the car has been driven for 10 mins or so.