Pulsating Noise 4Cyl Mode

[zibzer2]

2017 Silverado L83 5.3l
6speed transmission

Hello, not a new member but havent posted in a while and my old signup info isnt working so had to create a new account.

anyways, I have an issue with my truck and hope someone can spread some light on whats happening:

-when it goes to 4cyl mode it makes a clunk sound (but not when coasting or going downhill)
-when in 4 cyl mode and under load it makes a load pulsating sound (not a tick).
-goes away completely when in M5 gear or when plugging in an AFM delete.

Im guessing it is one of 2 things but im not convinced of either to be honest.
1. lifter stuck in v8 mode
2. torque converter shudder

realistically it doesn't seem like the TC as there is no physical vibration , and it never happens in v8 mode. but maybe?
and wouldnt a lifter make a sound whether with load or not?

any ideas let me know please. thank you!