Grand Prix 2006 NA engine pinging

Hi everyone, hope someone or many people on this knowlegeable board can help me with this. Every summer I experience this. The car will ping moderately on acceleration when the car reaches the midway point of the temp gauge. It does not ping when the car is cold. It will also not ping if I turn the heater on. By turning on the heater, the temp gauge will mostly stay at the 1/4 to 1/2 point. The pinging does not affect acceleration. I hear the pinging going up a hill or taking off from a stop sign but only if I am heavy on the gas pedal.
So far the MAF sensor has been cleaned with CRC MAF sensor cleaner, the TB plate has been cleaned with brake cleaner and the K & N filter replaced with a paper one. Spark plugs were replaced last year along with the wires. The plugs were the AC Irridium ones, wire are Parts Master.. Fuel pressure was taken and it is at 65PSI.
The noise is driving me nuts!

Is the car overheating? Losing coolant?

Any codes or EGR codes set?

Using 87 Octane?

No codes, using Premium all summer.

Car doesn't seem to be overheating and not loosing coolant. The temp gauge shows a bit over the half way point or just at the half way mark. EGR can be reset?

Why are you using premium? The 3.8 uses 87 octane.....

My mechanic said to use premium to see if the pinging goes away.

It's 4C degrees here in Montreal this morning and the car doesn't ping anymore since the weather is much cooler. Does anybody know why the cooler weather stops the pinging? I've read on the Grand Marquis section of this forum from other members and their cars stop pinging when either their car is cold or the weather is cold.