Subaru Outback AC Flowing Churning Noise

I purchased a used 2019 Subaru Outback about 2 months ago. The AC makes a strange on/off sound. It sounds like something is flowing or churning or even defrosting in a way. The sound almost sounds pressurized if that makes sense. I believe it has something to do with the evaporator. I notice the noise comes and goes more frequently when it is humid out. The noise will turn on for a couple minutes and then turns off for a couple minutes. It goes back and forth between on and off during the whole duration of the car trip. I have also noticed a on/off funky smell (sweaty sock or pond water). I do not smell anything sweet which I think would be the case if the freon was leaking, which was my first thought with the sound since it sounds pressurized or gas like. The AC does blow cold, though it does seem to be weak, though I have nothing to compare it to since this is my first Outback and it just might be that way. I have uploaded some video to see if someone one might be able to pin point the sound and what is causing it. The noise happens on recirculation and outside air. And it happens at any fan speed (though it is harder to hear at higher fan speeds). Only time it does not make a sound is when the AC is off and the fan are just blowing which makes sense but thought I would include that just in case.

Refrigerant is an odorless gas so it has no smell.

Has anyone checked if the condensate drain tube is plugged?


Thank you, that has not been tested yet. However, there is usually a pool of water under the car after driving so I am assuming it is not clogged?

That’s correct.

Has anyone checked the coolant level?


The coolant is about 0.5 inch above the low level line.

I’m wondering if air has gotten into the cooling system, and what you’re hearing is air and coolant rushing thru the heater core.


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Thank you for your thoughts on that. How would that be tested? I want to take the car to the shop, but I wanted to pin point the issue beforehand so that I am not just giving them free will to repair and replace things that may not be causing the issue. Have done that before with a different car and it was a very lengthy bill (and the problem still existed).

Ask the shop to bleed the cooling system to see if there’s air.


Great! Thank you, I will have them try that. Could that also be causing the musty smell? Or is that a different issue all together? I had read that if the evaporator is not working properly or got dirty in some fashion, then it can grow mildew which causes a musty smell. I had initially thought the sound was coming from the evaporator (since that is where the sound seems to be located, behind the infotainment center), and might be the reason for the sound and smell.

A musty smell usually occurs because the AC condensation rain tube is plugged. But also be caused by debris on the evaporator…

You can also ask the shop to treat the vent system with this product.

It’s formulated to remove musty/moldy odors from vent systems.


Okay great thank you! And if that does not work then I am assuming the evaporator would probably need to be replaced?

The Frigi-fresh should clean the evaporator if it has debris in it.



Here’s the kit the shop should use.


Thank you, so very much I will have them use that.

Also, has anyone checked the cabin air filter?


Yes I actually replaced it about a week ago. Did not seem to bad. Made a slight difference but there is still an on/off (occasional) smell. I replaced it with a carbon one.

Then I would hold off on the Frigi-Clean, and just do the Frigi-Fresh…

The cabin air filter should of prevented any debris from collecting in the evaporator.


From what I have read on a Subaru Outback site, the gurgling sound is normal on Subaru’s. A lot of owners from different years complain about it. this is what someone posted on the site…

The gurgling sound is normal…mine does it too. You will hear it more when it is not real warm outside but you have the A/C on to help dehumidify the interior…like when its raining and the temp is about 50-70 degrees F. You will hear it less when it is 75+ out. Subaru A/C does not use a compressor clutch to regulate the amount of cooling…it uses an expansion valve. The sound is just freon flow being restricted by the expansion valve. When there isn’t much gas flow you can hear the bubbling in the evaporator. Absolutely normal. Here is how Subaru describes how the expansion valve works.

• Sprays the high temperature, high pressure liquid refrigerant from the small hole in order to let the refrigerant expand rapidly to turn it into low temperature, low pressure mist.
• The refrigerant amount is adjusted according to the refrigerant vaporization condition in the evaporator.

Again, perfectly normal for systems with expansion valves.

Thank you for that insight. I was not aware that this could be a normal sound!

I haven’t seen a CCOT AC system in a vehicle for years.