I had a strange issue. I havent been driving much during the COVID-19 lock down, but I am not sure if that makes a difference. For 3 straight days, I made a few short trips in my 2015 Hyundai Elantra. The car is running fine, but when I first start it up I heard this strange gurgling noise, like boiling liquid. The car's temperature was fine and there was no issue driving that I could feel. I also noticed that driving seemed to have no affect on the noise. It would gurgle when I start, I drive for a bit and hit a stop light and the noise was still there. The noise did stop eventually, but I couldnt figure out why. I was sitting at the drive through at Taco Bell and decided to schedule a service appointment listening to the noise. On a whim, I turned my AC and fan on and the noise stopped and hasnt come back since.

Was that all it needed? Or should I still take it in?

  • It could have been refrigerant slushing around, it's a good idea to run your AC regularly just to keep it all working, sounds like you took care of it.
    – GdD
    Commented May 7, 2020 at 7:16

1 Answer 1


That gurgling noise is there is a "bubble" of air in the heater core. That will absolutely cause a loud gurgle whenever the engine is running.

  • Coolant flows thru your heater core all the time. They use a plastic door to divert air through the core (winter) or around the core (summer).
  • You have a problem in the coolant system. The system is not self purging all the air out.
  • My guess is you have a bad radiator cap. The cap on your vehicle has two seals. One used for high pressure relief, the other a flow back valve that sends coolant from the overflow bottle back into the radiator. Your flow back valve is shot.
  • Location of the cap: Radiator Cap on Hyundai
  • See photo and response to this question for more details.
  • Its also possible you are just out of coolant (in the overflow bottle), or the small hose between the radiator cap and the coolant overflow bottle leaks. Pls check.
  • These are inspections and inexpensive repairs you can do yourself. Note: do NOT open the radiator cap on a hot engine. In some conditions that can be quite dangerous. Best to do that work in the morning before starting the car for the first time of the day.
  • A coolant air bubble purge takes one or two days of engine warm up, followed by eight hour cool down... During the cool down cycle, coolant migrates from the overflow bottle back into the radiator.

Let us know what you find!

  • then why did turning on the AC and fan seem to fix it? It hasnt happened since.
    – Keltari
    Commented May 7, 2020 at 4:50
  • Aberration. My guess is thermostat opened at that exact moment changed the coolant pressure dynamics. But... those things I asked you to check take like 60 seconds. 1) radiator cap, all springs and seals working? See photo in link above. 2) coolant levels good in both radiator and overflow bottle? 3) overflow hose in good shape ( thats the small hose that runs from radiator cap to the overflow bottle. ) What do you see?
    – zipzit
    Commented May 7, 2020 at 14:27

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