1999 Ford Ranger Spark Knocking

If the Dealer has run out of options, I assume the check engine light is off? Check the cooling system by using a cooling system tester, pump it up to the rating of the radiator cap, or cap on the recovery bottle. if the prssure doent hold, it has a leak, and could be internal at a cylinder head gasket, it does not need to be a large leak to cause the ping... or buy a block test kit and test like this:
http://www.rxauto.com/demo.htm