car makes strange noises while parked.

when the car is in the garage, a few hours after being driven, it makes these strange "burping" noises every few minutes. erk. erk. erk.

I've listened, and I can't pinpoint where it's coming from, other than the front end. is it the coolant system losing pressure as it cools off, or something like that?

i figured it might be something like that, but it had me wondering.