Engine Knocking 23 1.5t MT

I have run a couple tests tanks now that it's 95 degrees everyday here in Florida... Once the car is heat soaked and the ambient temp is high, this car will knock under load at low RPM on 87 gas. I have to rev the motor and slip the clutch more to prevent knock.

If I fill the tank with 91 or 93, this issue completely disappears. Has anyone else noticed or experienced this?