Humming noise from under rear of truck but the truck has been off for 9 hours. Help

I went outside to check things out and I heard a humming coming from under the rear of my 2013 Tacoma BAJA. Sounded like a fuel pump but I've never heard it before, especially not when the truck has been shut off for 9 hours. I turned the key to the on position and it was still going, turned the key off and it shut off. Then I started the truck to make sure the battery wasn't drained and it started right up, shut it off and no sound. WTF??? I could hear the noise from 12 feet away.

The noise sounded like it was coming from above the rear axle but I know it can easily travel under there and mislead you from its origin. Any help would be great. I'll upload a video once I figure out how.

 

Your truck is doing an emissions leak test. Totally normal.

 

What the fuck!

 

I've never heard it do this before, I've had the truck almost 3 years now.

 

Agreed. Completely normal.

 

Another confirmation it's a normal function even if the truck has been off for hours

 

Like other said normal.
I got my ’07 in May of ’11 I have only heard it do it like 2 maybe 3 times.

 

I've heard my truck do it. I asked the dealer and they say it's normal. Meh.

 

Ok, just caught me off guard , I open my back door to my house and hear a humming noise coming from under my truck. LOL

I though maybe a ground wire for the fuel pump was corroded causing it to turn on or something. Glad to hear that it's nothing to worry about.

 

Everybody knows this. Evap. emissions checking.

Timing is everything. You just happened to be near when it happened this time. It may be another three years before you hear it again.

 

I have a 2015 Highlander, in addition to my 2013 Tacoma, and I have heard both make that noise at various times as I was walking in and out of my garage. The beer fridge is in there, so...

 

I have heard this on my 2013 one time. I do not know at what rate the EVAP system performs this test but it is normal.

 

" The beer fridge is in there, so... " You had a much higher percentage of catching it in action!

LOL I like the way you are thinking.

 

lol yeah i have never heard it on my 2013 tacoma... but just last night i walked out to the front gate past my dads 2016... and it was humming.... at first i was like...wtf, then i went o wait evap emissions test.

 

I think it comes on 4 hours after you shut the engine off. Not sure if its every 4 hours though