I've posted about this before, just thought I would bring it up again w/ a couple of things to add. Anyways, our 2000 Corolla currently has 152K miles. At around 100K miles, it developed a ping. The said ping, usually occurs at speeds of 55-70. From my readings, it seems this issue is relatively common to this generation of 1ZZFE. Since the ping began, we've replaced parts and tried various things. -Catalytic Converter/o2 sensors(check engine light). -PCV Valve. -2 MAF Cleanings. -1 Seafoam treatment(had to replace cat shortly after, related??). -2 ARX Clean/Rinses. -Multiple Fuel treatments. -Spark Plugs. Now for the things to add; when we travel up North, up to 7500 ft. elevation, the car doesn't ping much/ever. The car doesn't have an EGR valve, but does have a knock sensor. I've thought about the knock sensor, but it's pricey and hasn't thrown a code for it. I'm currently running MMO every tank or every other tank to see if it helps. Does anyone have any ideas on what could be causing the ping, and what I can do to try and alleviate it? Feel free to ask questions. Any input appreciated.