Loud rattling when rpms drop

[Will Back]

I recently picked up a 2006 Silverado 1500, the previous owner stated it was making a knocking noise. He pulled the transmission thinking it was the flex plate but didn't find anything.

I just put the transmission back in and noticed it only happens when engine rpms are dropping. Additionally, it mostly happens when in neutral. Posted below is a link to a video where you can hear the sound.

 

[CamGTP]

That to me just sounds like something is really loose. Like a skid plate, heat shield or some damn thing.

 

Crawl under the truck and have someone rev it, that is the only way to pin point the location of the noise.

[Will Back]

Agreed, I had a good look under there and I haven't found anything yet, I need to poke around more still. So far I've confirmed the transmission cross member is good, has a new trans mount, exhaust y pipe is all tight, and center support bearing is tight

[CamGTP]

I've heard of cracked flex plates causing a knocking noise, but that seems extreme for that and you should have noticed a huge crack in the flex plate when you bolted the transmission back together.

[Will Back]

Yea, I had a look at both the TC and the flex plate, they both looked fine on the outside before I put the trans back on. Still haven't found anything yet, double checked that all the exhaust bolts are tight

[John Matuszek]

Mine is doing the exact same thing and I have not been able to figure it out. Has there been any updates?

[PunchT37]

I have heard cats do this before. Core rattling inside.