Heavy Ping

I own a '97 Aerostar 3.0L 50K miles. The seems to run good, gas mileage is good always starts. I have noticed this year it will ping a lot going up tiny hills, "Florida hills". I will have to slow down to 45mph, I'm worried about damage. Then some times it doesn't do it or only slightly. I have added gas treatment too and doesn't seem to help. I read that there may be an anti-knock sensor that might be bad. The manual I have does not show one. So is there one? And if so Where will I find it?