I wanted to post my experience around this topic. My 2018 was also experiencing a rattling noise for 3-5 seconds on cold start which continued to get worse as time went on. I continued to video document the noise each morning. I took it in this last week to get diagnosed at around 12k miles (this started around 6k miles). I received a call from the dealer that they are replacing the VCTS, timing chains and all associated hardware. Parts are on order but will not be received until the end of the month, they are quoting a 2 day turnaround once the parts are received. Looks like they will have the truck for almost a month from front to finish. In the meantime they were gracious enough to provide me a loaner for the duration of the repair. I will update this thread when I get the truck back, hopefully this will fix the issue.