Ok, I left the car at the dealer yesterday, Monday, and just now got it. They had it for two days.
They were finally able to analyze the sound and see where it was coming from.
Something that holds up the struts in the back is what I was told.
I don't have the receipt with me because they're going to email it to me. At that point I assume I will know more.
I was in a hurry, it's the end of the day and I have a lot of work to do.
What's interesting is that the shop did the research via Infiniti and they found out this is only the second case nation wide of this same noise.
So they're documenting everything and have to make some special notes on the warranty because of the rarity of this, after that I'll get the receipt.
Will post what exactly it is at that point, assuming they describe it fully.