Yeah that definitely sounds a bit odd. Does it happen every time? Just went the car is still 'cold' from sitting? That sound almost sounds like a deeper version of the noise most of us 5.6 guys hear when we start our cars in the morning (https://www.youtube.com/watch?v=PvbM42SaFFA
) which is related to the engine being in closed loop vs open loop.
Did the dealer do a ride along with you? If not, request that. If so, who did they send with you? A Jr. tech or a Sr. tech?
I once had an issue where they sent the guy who washes the cars with me for a ride along (totally green kid) and he of course didn't hear anything. I then filed a complaint with Infiniti USA HQ and told them that the dealer is basically ignoring my complaints (I had the Elite warranty at the time) and within like an hour, the GM of the local store called me and promised to send me on a ride along with his most senior tech. Within 5 minutes of riding with the senior tech, he said, "Yep, something is wrong here" and we proceeded to do like 25 miles and 40 minutes. Once we got back to the store, he jacked the car up and immediately called me over (ended up being the subframe bushing or something along those lines).