I hear a humming sound under my 2015 Highlander when its not running

Because it is, it's the cooling fan, when it reaches a certain temperature it shuts off on it's own, this is normal.

When the car cools down from running or just from sitting in the garage when it is warm outside?

Mine does this as well. It is hours after the vehicle has been in use and the sound comes from under the passenger rear. I will forget about it, then walk out in the garage and hear it again. I really don't think it's the cooling fan when it hadn't been driven for 5 hours and it is 63 in my garage.

Had a similar problem. My service advisor was able to find the issue. Something to the effect of "Servo motor Making noise per TSB-0048-16". "Replaced servo motor per TSP and installed insulation sheet per TSB. Renationalized servo motor." It is covered by warranty. It seems the dealership may push back because they want to charge you labor but they shouldn't because it is under warranty.

Happened to my 2016 Highlander too after resting 9 hrs, in my garage,

I just bought a 2017 Highlander and it did the same thing after sitting in my garage for about 8 hours. The deal said they didn't know what I heard even though I had a video of it, not very helpful. Did anyone get this fixed and if so what was the problem

Our 2013 Highlander was making a similar noise from under the rear passenger side and I found this on page 25 of the owner's manual: "Approximately five hours after the engine is turned off, you may hear sound coming from under the vehicle for several minutes. This is the sound of a fuel evaporation leakage check and, it does not indicate a malfunction."

Similar to Alfie's answer above, Our 2016 Highlander was making a similar noise from under the rear passenger side and I found this on page 8 of the owner's manual: "Noise from under vehicle after turning off the engine" "Approximately five hours after the engine is turned off, you may hear sound coming from under the vehicle for several minutes. This is the sound of a fuel evaporation leakage check and, it does not indicate a malfunction."

This also happened to me with my new 2017 Highlander. I had to go into the garage around midnight. The car had not been driven for hours. I was investigating and the noise stopped as soon as I got in the car with the key in my hand. I have a call into my salesman to see if he's heard about this and if I should take it in to the dealer. I've only had the vehicle a couple of weeks.

See Owners Manual 2017 HIghlander - page 8 - "Noise from under vehicle after turning off engine".. explained well.

I just heard this as well in my new 2017 Highlander. When you hear the humming, how long does it last?

Its happened twice to me the longest i heard it for was 10mins. but not sure how long it was running before i heard it.

found the answer to this one on page 8 of the manual "Approximately five hours after the engine is turned off, you may hear sound coming from under the vehicle for several minutes. This is the sound of a fuel evaporation leakage check and, it does not indicate a malfunction"

Just noticed this noise (rear driver's side mostly) in my 2018 Highlander (2.5 mos old). The car has been sitting for approx. 7 hrs. Also have noticed (from the first week) a sweet perfumey smell from the rear vents especially when the heater is on. I think I found that it might be the AC core. I wonder if these two problems are linked. Thanks for all your answers and input.

So does it do it app. every 5 hours? Noticed ours after 10 hours. Is there some kind of sensor that tells it to come on?

This just happened to me.. 2017 Highlander XLE. Located sound from gas tank, opened gas cap and heard a loud depressurization, sound stopped and put gas cap back on. It is some kind of fuel tank pressurization issue - like GuruYH4Z8 mentioned...

Mine is making a humming noise too - kind of like an electric motor. Goes on and off. This happens even when I am driving! But I've heard it in the garage too. To me it sounds like it is the liftback. And when I open the liftback it stops!

This just happened to me with my 2018 Highlander. I went into the garage after the car had been sitting for about 5 hours and heard a loud buzzing noise coming from under the back of the car. I started the car and turned it off and the noise stopped. I've had the car for 11 months and this is the first time I noticed it, but I usually don't park it in the garage.

Thank you for the reference to page 8 in the owner's manual. Our 2019 highlander, less then a week old made the noise in garage 5 hours after driving for several minutes.

I have a 2019 highlander and it made the noise around 2 am. The vehicle was parked around 9 pm. The noise was coming from the drive rear side. Thanks for the reference on pg 8. I will look at my manual.

It appears its a consistent defect on all Toyota Highlinder year models - why dont Toyota fix it prior to selling them??

It is not a defect. The car is operating as designed. For “proof” that the car is operating as designed, I have included a photo of page 8 from my owner’s manual (2017 RAV4 XLE AWD):

I hear air flowing noise when i start the car even when the AC/ fan is off. Does anyone experienced the same?

Thanks for all the feedback above! That buzzing sound was going off 8 hours after driving. I popped all the windows and doors, it didn’t fix it. After reading it was the gas tank pressurization feature, I unscrewed the gas cap and heard a loud hiss of the air sucking in. After replacing the cap, sound is gone! Now I know where it’s coming from.

Our 2020 Highlander does it too, about 9 pm at night, in the garage.