91 gmc k1500 5.7 automatic

The noise comes from somewhere in the front half of the truck.
It only happens when accelerating just before a downshift (think merging on to highway). It doesn't happen every time I downshift/accelerate, but when it does happen, it's while accelerating.
I don't think it happens from a stop. It seems to only happen when at speed.
Goes like this: driving down the highway -> depress pedal to accelerate or pass -> NOISE (briefly for less than a second) -> truck shifts down. This whole process from hitting the gas to the trans shifting happens normally, but with this noise in the middle.

The noise: Always very brief - less than a second. The intensity/volume varies. Sounds like a grinding/slipping/rattle.

The transmission was recently rebuilt. The noise was happening before, and did not improve after the trans rebuild. It's hard to even tell if it's getting any worse. It MAY be, but very slowly.

When the noise happens, within a few seconds, the check engine light comes on every time. After turning the truck on and off again, the light is gone. The code is knock sensor, but there is no knock sensor. Like the wire is completely cut and removed. The code is also the only code that appears. (Other than the break light on the dash blinking)

It's been a mystery for a while. I've asked a handful of mechanics who all gave not even a guess. I'm hoping someone here will be able to help.