E53 making bubbling sounds, only heats intermittently

[SoonerBred1963]

As winter keeps coming around in Oklahoma, I still need my heater. For the most part, it is not heating. Just blows cool air, the temperature is running correct on the engine, but no hot air. Every now and then it will blow hot air, but for a little while and the quits.

Recently, I started hearing bubbling or gurgling sounds, that seem to come. right in front of the driver. Not sure what that means. I have checked the coolant within the last 3-4 weeks, will check again tonight. after it cools down. Thought maybe it might be low.

Any thoughts. appreciated.