Transmission makes a small rattle when downshifting

[XRipx]

2008 Silverado 2500HD with 6.0 gas motor and 6L90 transmission with about 93k on it.

 

 

I'm not totally sure if it's only happening when downshifting to a specific gear, but I can say sometimes, like maybe 25% of the time, there is a small "rattle" sound when downshifting.

It seems to originate somewhere on the driver's side, in the engine bay, down low close to the driver's feet. 

To be clear, it doesn't "clunk" and I can't feel the rattle in any way, it doesn't reverberate through the pedals or the steering wheel or the body and it doesn't affect the drive at all. The downshift will FEEL normal, it's just accompanied by this rattle sound.

 

 

Any ideas on this one?

[Mikebtte]

If the torque converter is knockin, you will be walkin.

Look at the torque converter bolts and make sure everything is nice and tight. Could be a rattle from a bolt or maybe a rock found its way in. Also the torque converter can rattle is failing internally, but that happens less often that rocks and bolts.