Some things that cause pinging or knocking-
-too lean fuel/air mixture (not enough gas for the air present)
-incorrect valve timing
-engine cooling system inadequacy
-faulty exhaust gas recirculation (EGR) valve
-high intake-air temperature
-driving in too high a gear
-excessive carbon deposits
Like what was mentioned above, I say try a bottle of Techron and a few tanks of premium gas. You don't have to permanently switch to premium gas, just use it for 3-4 tankfuls.
I gave up on taking my Rav to the dealer for just about everything, unless it is an easily recognizable problem or costs a lot of money is covered by the warranty.
Your pinging problem sounds like it belongs in the category of "Yes sir we checked everything out and couldn't replicate the problem".