Just found this on another site:
My '99 Passport has been pinging every now and then. I started reseaching until I read a detailed article written by a chemist who used to work at Mobil, formulating fuel additives for mobil gas.
This is what I learned, any brand of gas will have their own kind of residue that tend to build up unless you switch to another brand which cleans up the residue but the second tends to form it's own residue. It was recommended to switch to another brand every few months to prevent residue build up, I switch every fill-up. Bottomline, switch from one brand to another. I also switch from 87 to 91 octane depending on the presence of the ping and then back.
I used to pour chevron techron fuel system cleaner with consistent elimination of ping but it then came back after about 2-3K miles. Now with gas brand and octane joggling, pinging has been well under control.