What would cause a knock in my engine compartment?

every so often, I hear a knock-knock in my engine..It sounds like it's coming from the very front. this is a 2002 chevy venture van.everything else seems to be okay..as I said, the knocking only happens after it's heated up-maybe 2 hours. but it does not continue. I only hear it while idling.the engine has about 100,000 miles on it. any ideas, would be appreciated. i drive most days 200 miles.