This is kind of a tough one, due to the fact that engine noises are hard to diagnose over the net. What might be a knock or ping to you, could be a tick to me.
However, I can give you a couple places to start. If this problem only occurs while the engine is hot, you might want to change the thermostat and coolant, and also check the timing and see what that does.
If that doesn't help, you're probably looking at a more serious engine repair. You could have a sticky valve, a bad lifter, a clogged oil passage, or worse.
You should be able to isolate the noise by using a long steel or wood (preferably steel) rod and placing it on different parts of the engine while holding the other end to your ear. This should tell you approximatly where the noise is (top, bottom, left, right, front, back).
I hope this helps & good luck!
Ben
