Pinging engine

I have a 2000 F150 Ford pickup. Over the past few months the engine has been making a pinging noise when accelerating up hills, and occasionally the service engine light goes on and off while it pings and feels like it may stall. I tried different gas stations and grades of fuel, still pinging. My mechanic replaced all the coils, the thermostat and the fuel filter and it's still pinging.

What can cause engine pinging other then what I've addressed?

Thanks!