'92 Metro pinging...

My '92 Geo Metro with the 3 cylinder motor and 95,000 miles has recently started pinging quite noticeably. This happens even on the flats without hard acceleration. Sounds like a handful of gravel rattling around in the catalytic converter.
I keep this little car tuned and well taken care of. It's probably the cleanest older GEO around.
I checked the timing, which was right on the money, so I suspected the timing belt. I replaced this and the related timing marks were exactly where they should be.
However, the pinging was still very bad. I tried a tank of mid-grade gas, still no help.
I retarded the timing to 0, which is about 6 degrees before where it should be. This helped some, but the pinging is still there.
Things I've checked so far...

* no vacuum leaks
* plugs are cleaned and gapped correctly
* plug wires are good
* cap and rotor are good
* EGR valve, I think it's good
* Throttle Body Injector

One thing that is interesting.... the pinging doesn't occur until the car is fully warmed up. When it is cold, no pinging. Could that possibly indicate a lean running condition when hot?
Thanks very much for your help!!!

You said it sounds like things flying around in your converter? Perhaps your converter is plugged, and falling apart internally. If it's plugged, the exhaust gasses can't escape quickly, and can cause a number of problems. I would have it checked out. A universal replacement cat is fairly cheap.

Thanks for the reply, geozukigti...
Actually, I don't think the pinging is coming from the cat. converter, it's just the general area that the noise is originating from.
If I retard the timing to an extreme, this reduces the pinging, but not completely.
Any other ideas?
Thanks.....