Bubbling noise when heat is completely off

I have a 2003 2.4L Tacoma Pre-runner and when my heat knob is all the way to the cold side (a/c) it makes a bubbling noise. If I turn the knob just a little to add heat it seems to stop bubbling. With that said, I think I have some air trapped in my heater core which is causing the noise. I have refilled the radiator with more coolant but that hasn't fixed the problem. Is there a bleeder valve somewhere on the heater core to let air out or what steps should I take next to try and resolve this bubble noise?

 

If you’re feeling lucky. Squeeze the radiator hoses to burp out any trapped air.

 

I have tried that but it still bubbles. Could it be something else?

 

Might have a blown head gasket.

If the coolant level keeps dropping and there aren't any external leaks present, then the coolant has to be going somewhere

 

A test for exhaust gas in the coolant system is relatively simple and cheap. Talk to your local automotive technician.

If you are handy a test kit costs $30-$50 USD

 

Get the front of the truck pointed up hill so the radiator cap is the highest point of the cooling system.

Run the engine with the radiator cap off till the thermostat opens if you have air pockets this should eliminate them.

Good luck!

 

..yes but the question is, where is the "air" coming from? How is it getting into the system?

If there is a leak that will allow air in, then coolant should be coming out that same leak.

Make sure you have a good quality radiator cap and the expansion tank and hose is connected, unclogged and working properly. Pressure the system and look for evidence of a leak at the water pump, radiator, evap drain tube, etc

Check the coolant level every morning when cold before starting the engine. Right at the radiator cap. If there's room to add coolant, add some, drive it and keep checking. See if it steadily goes down over time.

If there are no external leaks and the coolant level still keeps dropping then an internal leak becomes more likely. Like a head gasket..

 

Yep. I remember getting that bubbling sound. Then all of this red dirt looking stuff started coming out of the overflow
This was on my 84 Cutlass. Ended up being a blown head gasket