Unknown rattle on cold start only. 2021 Chevy Silverado custom 6spd tran 77k miles

[Robert Paulitzky]

 

 

    It’s been driving me nuts trying to figure it out, I’m sure it’s not mechanical it or I don’t thinks so I have some mechanical knowledge. It sounds like a rattle, almost pebbles in a coke can. It will be very subtle on initial cold start if you have an ear you can hear it. On first acceleration it will speed up and then completely dissipate and not return until the following day. I used a rubber mallet on exhaust/cats and flex pipe could possibly be the issue. Sounds close to it. Just curious if anyone has this problem. Also if I let the truck warm up for 10 minutes it won’t make the sound at all. Debating on going to the dealer if I can’t figure it out even though the warranty it out. I have an oil change scheduled this Friday. Any suggestions would be appreciated! 

Edited October 14 by Robert Paulitzky