Quick revision to my previous post:
Through research of an exact answer as to whether detonation noise is from flame-front collision or the instant flame propogation hitting the cylinder walls I learned a little more info concerning your problem.
Basically, there are surfaces in your cylinders that are not smoth enough to disipate heat correctly so they begin to glow red hot. Usually it is carbon deposits on the piston and cylinder walls and the plug tip.
What was happening is a form of detonation termed "run-on" (fitting
).
So, when you pull the plugs also check for tiny specs of 'junk' on either of the electrodes. If you see any that usually means they are in the cylinder too. This is not a serious problem and definetly not worth an engine rebuild, just take care to keep coolant in the car and use a fuel injector cleaning solution (poured into a full tank of gas, usually found at a gas station) the next time you fill your tank.