[TerryH]

GM used to market "Top Engine Cleaner". Stuff was outstanding for cleaning deposits from the valve train and combustion chambers.

Modern cars automatically compensate for octane by retarding timing. You could have a bad knock sensor/computer, carbon build up, or worse case a partially blown headgasket. If it's blown between cylinders or into the valley, you won't necessarily get the exhaust steam or water in the oil. You could even have a clogged cat when the exhaust is retricted. Also could be fuel delivery related and knocking from lean mixture.

Pretty tough to guess based on limited info. I would recommend getting it looked at before you knock holes in a piston or damage bearings. It likely will only get worse when the weather gets hot.