Has only been happening for the past 6 months or so.
Ignition:
I'm with cdogg on this. Ping at idle would not come in 30 minutes after starting, warm up, running that long, now it pings? No. It would be ignition if say you had excessive heat coming off the engine. Idling at a light would be worse. So you are not complaining about heat, nor have you mentioned it is hard starting = Not Ignition.
30 minute window and 6 months it started with this tick/knock:
I agree with the rest of the input. 30 minutes in you start hearing noise? You'd think a failed lifter not holding pressure would knock on startup. Heats a different story. 30min in and it idles so no valve is tight it keeps the valve(s) open. No rocker chatter upon startup where it's too loose. No cold knocking, but some 30 minute range this is now on the raggedge of ticking at idle? Piston knock would sort of knock the small end of the rod against the wrist pin. It has to stop at top dead center so when it goes down it ticks, swings back up from bottom dead center, and as it comes around and ticks again you hear both ticks. Only thing is, rod out of tolerance is going to get louder 6 months goes by = Bottom or top end noise is to video a cold start and let it idle. Warm it up until it makes noise, where you'd try to find the loudest area you think it's coming from and video that area.
Plugs:
Where are the 3mo old plugs? Look at those and describe them or pull the new plugs and note who is not matching each other, etc.
