I've noticed the exact same thing with my '05 (only 66k miles though, and no engine work).
Sooo, what do you think was the real cause of the noise? The timing belt guides? Idlers seem an obvious choice, and I'm on the verge just replacing them and the tbelt. No one has touched the inside of my timing cover since the factory (that I know of).
I park mine in the garage, so the occurance of the noise is even less, but happens in the cold.