i just checked my maf sensor a few days ago and blew compressed air in it. i reinstalled the maf and voila! no more pinging sound hehehe. i guess when i cleaned my maf using electric contact cleaner a few weeks ago either i didn't let it dry thoroughly, or maybe installed the maf incorrectly as i noticed that i could easily remove the sensor(pretty loose after removing the screws), i suspect that a little air must have entered thru the hole which confused the ecu into leaning out the mixture..hence the pinging sound.(i hope i know what im talking about as i myself am confused hehehe). but whatever it is i did, im just glad that everythings back to normal now. just for good measure, i filled up with petron xcs a while ago(from the usual petron unleaded)