Lexus ES300 is making a brief knocking noise after I rev the engine? Any ideas?

I have a 2000 Lexus ES300 which I just noticed makes a brief knocking noise after I rev the engine or when I turn the car off. The knocking lasts for like 2 seconds but is only audible after the RPMs come down from like 4 to 1 rapidly. Just had the toyota guys look at it but they couldn't find anything. The car drives fine and has no other problems. I wonder if I should just take it in to the Lexus and see what they say. The sound seems to be coming from the engine block. Any ideas??