I'm pretty sure this is the head gasket on it's way out. It take the oil cap off when I start the engine, and there's a very audible gurgling sound, similar to when the last bit of water goes down the sink. Only it's continuous.
There's also a lot of white smoke coming from the exhaust after a minute of idling.
btw, the car has sat for six months, done about 10 miles in five months, though it has been started regularly.