Do you drive your engine ultra-conservatively?  If so, it could be full
of carbon (a common source of pinging) which needs to be removed.  The
carbon continues to glow after the combustion stroke so during the early
part of the next compression stroke, the glowing carbon ignites the
air-fuel mixture prematurely. If you think you might have been driving
that conservatively, spend a few days driving it ultra-aggressively by
revving to high rpms under wide-open throttle  as much as possible to
blow the carbon out.  Yes this is a legitimate technique that I didn't
just make up.  There's another way which involves SLOWLY trickling 16
ounces of water into the air intake system as the engine runs to
scavenge the carbon deposits out... which is a highly effective method
so long as you don't pour the water in faster than a slow-(SLOW)
trickle.  I take a few minutes to pour in 16 ounces... which leaves the
combustion chambers sparkling clean, really... without any damage so
long as you take your time as described. :)

Jerry