Who's knocking?

Slight knocking sound under slight acceleration around 1500-2300 RPM. More prominent with the A/C on. Really starting to bug me. Any clue as to what can cause this? It almost sounds like plastic knocking together (yes, I check everything to make sure it is tight).

2013 OR 4x4 v6

 

Try filling up with 89 octane (mid-grade), odds are that'll solve it for ya. Seems to be an increasingly common issue.

 

^^ This. I did it and it worked.^^

 

Roger that. Just happened recently in the hotter months. I know the manual says that some knocking may occur and to not be alarmed, but seriously...

 

Just went through 2 tanks. One was 93 and the other was 89. The knocking subsided. I filled up with 87 earlier today and it came back. Not bad, just on acceleration... slowly. What gives? Why can't I run 87 in my truck without it knocking?