yup same here in my 2000 amigo.
On again off again. Chevron techron concentrate (9 bucks at Target) gas additive every 3k seems to help to keep my ping away with 89 octane. Also, make sure your PCV is clean and your EGR valve, tube and port. My dealer was never able to find a newer program. I filed a consumer complaint at:
http://www.nhtsa.dot.gov/cars/
The funny thing it is listed on their site now to (click on consumer complaint, select isuzu, amigo, 2000, engine gas. and sure enough my info is in their. After I did this they called me and talked a bit and then Corp Isuzu mailed me a couple of photo copies of the manual that states the knocking is normal underload and said have a nice day.
I'm not to freaked out about it. For the most part it has stayed away as long as I put in 89, techron every 3k and clean the EGV parts and PCV once a year. My previous truck had an engine ping for about 100k I sold it at 185k and two years later I still see it cruzen around.
Also, I read a recent post, I think here, that state some isuzu's have champion plugs which seem to have add to the pinging problems. Might be worth your time to swap plugs to (NGK). I have not looked at mine yet (45K), but plan to yank them this fall.
good luck!
-Ryan