I have a 2000 323i and lately after it has been parked for 4+ hours when I go to start it it makes a hissing rumbling noise from under the hood. It sounds like boiling water almost. Revving the engine does not make it stop but after waiting about 2 minutes it goes away. I can still drive it when it makes that noise but I don't. My car has no other issues besides that. Any ideas?