Buzz or small pump noise in a parked, not running 2018

Tonight I walked through my garage and heard a humming or buzzing noise coming from my 2018 Tacoma TRD Sport. It is cold here, probably in the 20's in the garage. The truck had been parked for several hours. I walked around the truck and believe the sound was coming from under the bed on the drivers side. I heard this once before recently, also on a very cold night after the truck had been parked for several hours. I made a quick search and was pointed to this small notice in the Owner's Manual:




I've owned this Truck for 3 years now and only noticed this sound twice, and both in the past weeks during cold weather here in PA. I'm happy I was able to find an explanation relatively quickly. And, I'm surprised I hadn't heard of this noise from any of my reading on sites like this one before. Maybe I just never paid attention, but I couldn't find any reference to it with search. I post this here as a note in case others are hearing it.

 

When I got my brand new 2020 I actually read the entire owners manual. A dry read to say the least but with some surprising info such as the strange noises after parked. So I was actually aware of this.

 

It seems somebody asks about this once every couple weeks.

 

That system started all the way back on the 2005 Tacoma so it's been brought up a time or two.

 

The weekend after I bought my truck I was out camping, sleeping in a bivy sack next to the truck and that noise started around 2am. Thought maybe I had left it in accessory mode or something and might be running down the battery, so I got up and checked the truck over. I'm surprised sales reps don't mention this when you pick your truck up, since it is definitely unexpected, and most people don't read the manual.

 

18 Highlander and 20 Tacoma both do it.
Just assumed it was them growling at the Ford work truck.

 

Sorry, I have been reading and posting in this and the 2Gen for over 10 years and never saw a post on the subject. I searched, but I suppose I chose poor search terms and found nothing. If you have an extra minute, could you share a link to one of these posts? I'd like to see how it was described, and why my search never found any of them.

 

https://www.tacomaworld.com/threads/truck-off-fuel-pump-running.743116/

https://www.tacomaworld.com/threads/electric-whirring-noise-in-rear-of-truck-after-shut-off.707463/

https://www.tacomaworld.com/threads/not-so-mystery-noise-solved.696195/

https://www.tacomaworld.com/threads/evap-pump-noise-normal.655469/

https://www.tacomaworld.com/threads/air-compressor-noise-while-truck-is-parked.647818/

https://www.tacomaworld.com/threads/noise-in-left-rear-fender-well.636512/

https://www.tacomaworld.com/threads/humming-buzzing-noise-between-bed-and-rear-seats.545960/

https://www.tacomaworld.com/threads...ype-noise-coming-from-under-rear-body.519870/

https://www.tacomaworld.com/threads/strange-electrical-noise-from-underneath-the-truck.477894/

https://www.tacomaworld.com/threads/help-canister-assembly-running-while-off.427708/

 

pretty sure that's the real reason.