I own a 2001 GMC Jimmy and when we start and run the vehicle at times it sounds like water is russing and bubbling and gurgling in the dash? Is this a heater core problem. I recently had to add coolant due to it being low, there was no indicator light to tell me it was low. I just decided to check the resevoir and found it empty and then check the radiator and found I could not see any fluid in it. I added fluid and the temperature is not registering that it is getting hot. We also smell a very fishy smell coming from the engine at times. I have 72,690 miles and it is now five years old. I have not changed out the radiator coolant as of yet.