Do you happen to have the TSB# for this? My truck (2018) is doing something similar to this. When it is acting up, I can hear a sputtering, whomping type sound from the drivers side. I have been searching this forum for an answer and have found different solutions from different people all with seemingly similar problems.
These are the possible solutions I've seen thus far:
Loose exhaust clamp that is near the catalytic converter
Bad vacuum pump
Bad steering column/firewall seal
Torque converter locking up (this thread)
Exhaust flapper valve (I have mine permanently open which stopped the ticking sound I was hearing at idle...it sounded like a loose heat shield)
I also suspect that it may be AFM related based upon other things I've read.
I drove into work today in "L5" to keep the AFM from turning on. The issue I'm experiencing was seemingly absent. I'll pay attention on the drive home today and see if its still gone. If so, I may just disable my AFM. I'm not a fan of taking my truck to a dealer and letting a technician drive it all over tarnation hoping to find some intermittent problem. My past experience with dealerships is if you want results, you have to arrive armed with all of the information that you can gather to avoid being given the runaround.
I must admit, I pretty annoyed with a new truck doing this with less than 2000 miles on it.