humming noise when engine is not running

Definitely sounds like the fuel pump. Probably the relay is sticking. Have your Toyota dealer check it out and fix it for free. Hope that helps! Jim

I am sure some dealers would love for you to bring it in for "required" service. However I just heard this sound under my 2017 Highlander (owned it for 18 months and first time I had been standing next to it in a quiet garage 5 hours after it was parked). Sounded like very small compresser under car behind back passenger side wheel. Checked google and after reading through all the "experts" with definite answers to the question (from malfunctioning fuel pumps to normal cooling fans), I found the real answer. Which is, "a perfectly normal noise of a fuel leakage check that happens approximately 5 hours after your car has been turned off". It sounds like this may be the same sound you are hearing. Check the first page in your owner's manual, after the contents, in the section "For your information" (page 8 in the Highlander manual).

I have a 2019 highlander and it is doing the same thin. Thanks

its the pump inside the charcoal canister doing a self test

Thanks for the info guys have the same thing with my 19 tacoma ...

1 year old trucks have a warranty people!

Strange that I had a 2016 Tacoma Off Road (4x4) and it never made the noise ... Got a new 2020 Tacoma Off Road (2x4) and it makes the noise

I have a 2021 Tacoma I just bought on Christmas. Walked by my truck on the driveway and heard the noise and co fused the heck outta me. I hope it’s just testing itself. That’s weird

I just heard the fuel pump in my new 2020 Tacoma, yes alarming when your vehicle is parked in a garage attached to the house. Put the Taco in accessories & off the pump stopped. Now I can go to sleep at least! Thanks for information.