Rattling sound from engine/transmission

[cgalpha08]

Hey all. 2015 gmc sierra 1500 slt with 6.2 and 8 speed w/126k miles

The other day i noticed this odd metallic rattling sound coming from where the engine meets the transmission, that general area.

I cant say for certain that it increases with revs, hard to tell from the cab. Sometimes it's there right on startup, other times not.

 

Here is a youtube video of it

https://youtu.be/K_kR47dC5PE?si=LPfcTWPIm7XwrZlH

Edited February 23, 2025 by cgalpha08

[BO TIE 1]

How's it drive? Is it shifting funny?

[cgalpha08]

6 minutes ago, BO TIE 1 said:

How's it drive? Is it shifting funny?

Seems to drive fine, just makes that noise. Doesn't over heat, shifts fine. 

 

I think it increases with revs, but the motor gets too loud and drown it out to be able to accurately tell. 

[BO TIE 1]

Could possibly be coming from the torque converter. Check the trans fluid for any metal shavings in the fluid. Do you have a check engine light on? 

 

[cgalpha08]

12 hours ago, BO TIE 1 said:

Could possibly be coming from the torque converter. Check the trans fluid for any metal shavings in the fluid. Do you have a check engine light on? 

 

I haven't had a chance to check the fluid. I did have it serviced last August, they said everything looked normal, clunk is normal, typical dealer bs. It still has an extended warranty, so I'm not inclined to mess with it until it gets bad. If there was an easy way to check the fluid I would, but since gm removed the trans dipstick, harder to do that imo.

 

I will say the sound is most noticeable under load. If I coast, I can't hear it, only when I get in the gas do I hear it. 

Edited February 24, 2025 by cgalpha08

[cgalpha08]

So i did some tap tapping underneath the truck. When I smack the exhaust I can hear a rattling from the second cat (circled in red).

So good news as it's not engine or trans, just some bad exhaust.

 

[CamGTP]

That cat can be cut out if you really wanted to do that. It's not there for anything other than meeting an emission standards. It's not monitored by the computer at all.

[red44]

I had a truck with cats that sounded like a maraca band at stoplights until I figured out the heat shield was half-rusted and vibing like a tuning fork. Your description is exactly what mine did—random metallic chaos that’d come and go like my motivation to fix it.

 

Since it’s the cat area:

• Heat shield: 90% chance it’s that stupid shield rattling. Get under there and wiggle it—if it moves, either bolt it down tight with washers or rip it off and yeet it into the sun.
• Internals: If the shield isn’t the culprit, the cat itself might be crumbling inside. Tap it with a rubber mallet (gently!)—if you hear loose chunks, congrats, it’s time for a new cat or a hypothetically hollowed-out one. 
• Exhaust hangers: Check if the exhaust is sagging and tapping the frame. A $5 hanger from AutoZone might save your sanity.

Either way, glad it’s not the engine/trans! But fair warning: that rattle will slowly drive you insane. Mine got so bad I started hearing it in my dreams. Good luck!

Edited February 27, 2025 by red44