Re: 2002 Explorer 4.0 pinging
First, see what code comes up. A local autozone can pull it for free and they may even elaborate on what the code means and how to go about fixing it. Something as simple as an O2 sensor perhaps. A more serious case would be engine misfire causing the pre-detonation, or 'knock' sound. Get the code.
Basics: Are you going through any fluids (most likely oil)? Oil consumption can lead to pinging. Cheapest thing to try to get rid of it is use your current fuel until you're on fumes, then try a higher grade. If your running 87 octane....move up to 89 or 91. Run a good ten gallons as some of it will mix with the lower grade fuel in the tank. Running a good (generally expensive) injection cleaner and changing out the spark plugs may also help. There really is no set cure for engine ping; or knock. I have it on my Durango (Dodge..I know), but that engine doesn't have knock sensors so it's hard to cure 100%.
'00 Durango R/T 360ci 290hp (modded); 138,500m
'06 Pontiac G6 GT 3.5L 220hp; 44,000m
'12 Chrysler 200 Limited 3.6L 283hp; 13,000m
'99 Taurus 3.0L 2V Vulcan 145hp; 154,300m - Traded
Amsoil in all vehicles!