2004 2.4L ES Pinging (spark knock) under load

Hey folks. My 2004 Galant 4-cyl just started pinging pretty bad under load. Sounds fine at idle, though the idle drops pretty low after the engine warms up. There are no CEL codes.
The idle is low enough that it almost dies when AC is on and it's in gear.

I replaced the spark plugs. Coolant temp is normal. I also cleaned the MAF sensor elements. Mileage is about 115K miles.

The low idle started about a week before the spark knock. Never had this problem before.

any help would be greatly appreciated...

Are u sure its pinging? hard to believe since im a mitsu mechanic. i would check fuel or timing.

Well, it definitely sounds like pinging. A loud, dry chatter when engine is under load. I would think it was a busted valve spring or something, but it still has plenty of power, and it idles nice and smooth when cold.

Metal dragging noise? Possibly loose exhaust heat shield. should be able to duplicate this problem while power braking.