This is from page 6-3 of my 1995 Geo Metro owner's manual:
"If you're using fuel rated at 87 octane or higher and you still hear heavy knocking, your engine needs service. But don't worry if you hear a little pinging noise when you're acceleratring or driving up a hill. That's normal, and you don't have to buy a higher octane fuel to get rid of pinging. It's the heavy, constant knock that means you have a problem."
Sounds like you're vehicle may be operating as designed.
Mike