555ci Engine Knock Sensor going off but only at Cruise
 

Like the title says, I am trying to figure out how concerned I should be that my knock sensor is going off, but only at cruising rpm 3700-4400. I pushed the boat to wide open and the knock sensor immediately stopped lighting up and it revved out to its normal max rpm of 5500. I am carbed and have an afr sensor to confirm that my afr's are all in good shape. 12.8-13.5 cruise and 12.0 wide open. All fuel, oil temp, oil pressure, water pressure, and water temp gauges are reading normal. The spark plugs are also new. Idea?! Thanks for the help

I am far from an expert, but are you running close to the 13.5 in that 3700-4400 rpm area? That is pushing it for normal AFR cruise numbers IMO.