Knocking/Pinging with 87 Octane Gas

2000 Mazda, Protege LX

I have a 2000 Mazda Protege LX with the (1.6L DOHC) ZM-DE engine, and since it was new, I detected a noise at aprox. 2,400 rpms in 2nd gear, at that time my father was the owner and he at the next fill up he refilled with 93 Octane Gasoline and the noise disappeared, now i have that car and i have to do the same, i have other cars with no problem with 87 Octane Gasoline, I checked the engine block and that car doesn't came with a Knock Sensor, i found the plug where it is supposed to be, I used colder sparks it lowered the noise, and the noise came when the engine its at his operating temp, when is cold you could race the engine with regular and works just fine.But for me is not worth it because is not a sports car, is a commuter car, it should run fine with 87.
2002 MP5
My Protg does the same thing but, it goes away when I fill up with 93 octane like you mentioned. I will be paying close attention to this thread to see if there are any solutions. My protg has been doing this for a long time and I thought it was just crapy fuel that is why a changed to 93 octane. Thanks for bringing this topic to light.