2017 GMC Rattle at cold start

[JLock]

I have a 2017 GMC Siearra 1500 with the 5.3L & 8 speed transmission. There is a rattle noise that you can hear at cold start. I have seen posts on this issue and answers seem to be all over the place. When my truck sits overnight or while at work all day, there is a rattle that sounds to me like it is coming from the exhaust. I haven't gotten under the truck to see exactly where the sound is coming from. It typically lasts about 5-10 seconds. If you immediately put the truck in gear after startup the rattle goes away. Any info on exactly what this may be is appreciated. I have seen suggestions of torque converter, heat shield, hole in exhaust, flapper valve, and bad catalytic converter. Can anybody narrow this down to one of these issues?

[rlcole321]

Yeah could be any of those, your just going to have to "Climb Under" and locate it (my money is on the cat)

Roll Tide!!

[Dphelan]

Not the cat. I just had mine done because that’s what the tech said but it’s still making that noise. And its worse. It costed about $2,000 out of pocket because my warranty was good for 80,000 is what the GM place said.

[Hired Goon]

Not sure if it's the same problem but I have something inside my muffler that when I tap on the muffler with my hand hard enough something inside rattles. Sounds like a possible baffle broke or bolt. I'm just living with it

[sk]

It's definitely coming from the exhaust flapper valve.  It was a known issue years ago and I got mine replaced under warranty.  They came up with a new design (moved from after the cat to before) but even that one rattled after a few years.

 

You can check by finding a square or round knob sticking out of the exhaust line near the cat.  Lay underneath while somebody starts the truck, then grab the knob and the rattle should stop.  At that point you can clamp it open with a hose clamp but when your truck switches between V4 and V8 mode you will get a terrible "rumble" kind of like a helicopter flying behind you.

 

In my case my AFM lifters failed (a different LONG story) and I decided to do a DOD delete.  Once you do that the truck stays in V8 mode all the time so I just tack welded the flapper open.