I have a 2005 Ford Freestar still under warranty. The valves have been pinging since about 15,000 miles. This is not "valves knocking" - the sound you can hear when the enging is idling, this is the pinging sound the valves make when you put bad gas in, for example or run too low an octane. I've been running 89 octane in it and the engine still makes the pinging noise, not only on inclines but on flat roads, too.

The dealer wants to replace the Mass Air Flow Sensor. Does anyone know if this will fix the problem? Any comments would be appreciated.

Hopefully the Dealer will figure out your problem. The PCM usually controls ignition to prevent spark knock. I do not see how the MAF sensor will affect this.

do you live in the mid-west where they use have ethonal?? Could this be causing your problem??