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.