There have been several posts concerning this noise that came with the LGZ V6 engine that GM started using in 2017 through 2019 and probably beyond. Mine occasionally does it. Conclusion is that it is normal and nothing to be concerned about. Supposedly it is a timing chain that needs to get self-adjusted and lubed before starting it's morning journey and is a little sluggish without it's coffee in the morning. Ooops, that is me.
I get that everyone once in a while. I know exactly what you are talking about. Normal to me and not a big deal. I have 2018 V6 as well. It's a start up rattle for like 2 to 3 seconds or so. I wouldn't be concerned about it.
It is the cam phasers on the ends of the four VVT cam shafts.
Every once in a while, when the engine is shut off, the locking pins on one of the phasers don't get to home position. On restart, there is a brief rattle as the phaser gets locked in by the pins seating properly.
YouTube has some videos of several makes and models of VVT engines doing it. It is not just the twins 3.6 liter.
The prevalent opinion seems to be that it won't hurt anything.
Actually GM released literature on some of the sounds that may concern owners with these high feature V6 engines, noises that owners may not be accustomed too and it turns out on cold start the new style gear rotor variablr oil pump may produce a brief rasp noise at cold startup due to oil drain back and air being present in the assembly and that dealers are not to attempt any repairs and this is considered normal. This is common but most people dont remote start and being they are not outside the vehicle when the key ignition start they dont even know it happens, I have asked a few people to cold start while I smartphone video recorded for them and bam showed them it happens to their vehicle as well.
The short cold start rattle is common, if it lasts for more than 10-20 seconds it could be a defective lifter, that happened to me, it was difficult to get GM to look at it but when they did they found it and fixed it, now I get a cold stat rattle once a month at most.