Annoying metal rattle sound but only at 48-50 mph

Hello all. I have a 2021 CM with about 15k miles on it and I'm having trouble locating the source of an annoying rattle. The crazy thing is that the rattle only rears it's ugly head when I'm at a constant speed in the 48-50 mph range. As soon as I slow down or accelerate out of that mph range it goes away. It sounds like its coming from directly underneath the truck. Definitely not coming from inside of the cabin or the engine bay area. Doesn't sounds like its coming from the wheel/brake areas either. Its hard to describe the sound but if I had to it sounds like a thin metal/tin sound. I know this is a shot in the dark but hopefully someone else had a similar experience and found a solution. Next step is taking it to the dealer but I'd rather avoid that.

 

Does it rattle if you rev it in park?

 

Or if you shift into N between 48 to 50 mph

 

Nope, only in that 48-50 mph range. Craziest thing...

 

Maybe a heat shield vibrating?

 

If I shifted into N the truck would start to slow down right away and the noise just goes away. Same if I let off the gas. Rattle only happens if my foot is held constant on the gas on a somewhat level grade. Any sort of acceleration or just let off the gas any it goes away.

 

That was my first though. I crawled under the truck and checked all the heat/exhaust shields and didn't notice anything loose. Maybe I'll crawl under again and be a little more thorough.

 

Have you tried S mode. See if 48 to 50 in 4th gear 5th gear and 6th gear produces rattle in each gear

 

Is it possible its coming from your engine bay on the firewall? There is a harness that has been known to make noise against the firewall.

 

I have not. Seems like the rattle only happens at the highest gear for that speed (assuming 6th gear) when the rpms are at the lowest (1100-1200 rpms).

 

Could be but it doesn't really sound like its coming from that area, more from the undercarriage. But at this point I'm not ruling anything out. Do you have anymore details on the location of this harness?

 

Wish I had the truck right now to take a pic for you but its on the firewall, by the passenger side behind the engine, i beleive straight back form the intake tube of the airbox. There is a decent size wire loom going into the firewall.

 

Cool, I will give it a look and see if anything feels loose.

 

I think a zip tie will take care of it, the harness loom itself would slap against the firewall.

 

I did exactly this to fix the harness issue that @MadMaxCanon is describing. I’ll take and post a picture when I get a minute.

 

Pretty obvious but make sure your muffler pipe bolts are tight.

I remember years ago on my 2014 all 4 of the front ones vibrated loose..

 

I forget what that harness goes into, is it the PCM?

 

Not 100% sure. Attached pictures for OP.

 

Yup thats the one, might even be the ECU?

 

Zip tied it 3 years ago. Quiet since.