what is most common cause of pinging ?

You may want to know what type of fuel you are using and the octane rating. I made the mistake of stopping at a gas station I wasn't familar with and ended up having water in the fuel. On your vehicle, you should be using only 87octane rating. Have you recently had any repairs to your van? Sometimes the wrong spark plugs or incorrect plug gap can cause this problem. It may be time for a tuneup. It could also be as simple as a loose or cracked spark plug wire. Other problems could be carbon buildup in the cylinders,or ignition wiring problems. Hope this was helpfull. Goodluck.

there is a valve called the egr valve that is controled by the pcm. if the passages are clogged, or restricted,, it will result in constant pinging. Roy

I own a 2006 Ford freestar with the 4.2L and have driven the Mercury version. Both ping between 3800 and 4200 (peak torque and Peak horsepower) when under moderate or more acceleration, consitant RPM and ping stops. My van has done it from day 1 (first noticed at less than 5000 miles) in the first 2 years of owning I had it in the shop several times (most mechanics where not paying enough attention or not reading the details in which to locate the sound). I believe I have determined the cause, as there is one twist. Under 40 degree air tempature the ping does not happen. It is my understanding that the Mass Air Sensor is not utilized to calculate fuel curve under 40 or 50 degrees, and that Ford has leaned the motor out so far that during its peak output, where it is burning every drop of fuel the fuel curve is just a hair to lean. And she still pings to this da, even after a carbon clean-out with seafoam at 60K miles.