My 3.9 L equipped '04 Freestar pings as well and this is the 2nd Ford in a row that pings during light throttle loads. My 1993 Ford Escort that I bought new did it too. I noticed that when I changed the spark plugs on that car, it would stop pinging for awhile. When it started to ping again, I'd change the plugs and it would be good for awhile. I kept that car for 189,000 miles and 13 years before I finally sold it 1.5 years ago.
On the Freestar, it only does it every now and then. I have yet to try and replace the plugs but plan to do so when I hear it pinging again. Currently, it has 58,000 miles on it. | 
