I guess the easiest thing to do is to go to AutoZone and pick up some octane booster and try that out or maybe there is a station that carries higher octane at the pump. If the knock goes away then it is a mixture of the 91 octane is crap (which is true for most of the country) and the tune is too aggressively timed.
If the knock does not go away with the octane booster, then there could be some false knock caused by metal on metal banging or some engine mechanical issues. Do you have a catch can? I have heard that too much oil in the combustion chamber causes knock.
I had this issue with my previous car. I knew my tune was pretty close to spot on and I would still be getting five or six degrees of knock. So, I found a local station that carried 100 octane at the pump ($7.50 a gallon) and tried that out. My knock immediately went away. Then I made the switch to running E-85 full time, no more knock!