I gave this problem some thought while trying to get a Saab APC boost controller with knock sensor to perform the way I thought it should. I wondered if some engines were just noisier than others? Noisy enough to affect a knock sensor. Seems to me that bad wrist pins, piston slap, or even out of adjustment valves could be a factor in internal engine noise. Luckily before I tore everything apart I found the right adjustment proceedure for the APC knock control and solved my problem but I still wonder, especially about valve adjustment. It seems to me that if you have replaced the knock sensor then the problem has to be in the ECU or the engine itself.