Spark knock.

[19-Silverado-98]

I've noticed it's mostly in overdrive. At 50. When I go to accel without it downshifting.

 

[sewlow]

Vortecs have a habit of eating the gear on the distributer. My '98 was suffering from the same probs as yours. Pulled the dist. & the teeth were knife-edge sharp. That caused havoc with the timing & made it knock under load. $80.00 for a new dist & no more knock.
The gear itself is not replaceable! Thanks GM!