Carbon buildup or bad tune?

I bought my 03 GT 5spd last year and ever since iv had intermittent problems with the engine. Sometimes when I shift I can hear it ping (marbles sound) when I shift some what firm about midrange rpm. Then some days when its really hot (100 degrees) it runs better and with more power. So im thinking its lean right? Then some days when its cold (30-35 degrees) its runs well also. It starts fine ,sounds good and other then this has no problems. Im thinking maybe someone before me changed the timing with a programmer or I have a lot of carbon deposits holding in heat causing the ping?? Iv tried the usual stuff, Cleaned the mass, throttle body, reset the computer many times. No CEL. Everything is stock except K&N filter and Mac cat back. Any thought or ideas welcome.

Oh and im not sure if its normal or not but throttle response is pretty bad. In neutral just reving the motor there is about a 1 second delay from when I hit the pedal to when it rev's. The harder I press the gass the more the delay.

Thanks for any help guys!

I removed the kick panel and it dosnt look like the computer has ever been removed. I dont think its chipped but maybe its been reprogramed for 91 octane witch iv tried. 91 makes it run about the same power level and the pinging is not consistant enough to be sure if it still does it on 91.

Where is the pickup sensor for timing? Maybe its loose? Iv looked for it on the crank pulley but I didnt see it.

What other sensors can I check or clean that may effect A\F ratio and timing other then the mass? Thanks..