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.
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