Weird electronic “hum” while parked

Ok so it’s starting to get “cold” here in Texas... Yes I know I know, spare me the guff Yankees. Anyways I’ve noticed either after sitting for a day or two and more often since it’s getting cooler outside that the truck doesn’t spin over as fast as it did when I first put the battery in (2 years ago). I attributed this just the first cold snap of the year, better get used to it...

Today while doing some woodworking in the driveway with my wife we both heard a soft hum or electrical sound. It almost sounded like a home AC running outside, but a street or two over. Paid no mind to it at first. After turning off the power tools another time I heard it again, but I was standing closer to the truck. I could put my head near/under the rear fender and it sounded like a small pump or something was running. Wouldn’t guess the fuel pump would run or prime with no key in the ignition, but it was definitely coming from underneath the truck.

I started the truck once and let it idle for about 20 seconds and shut it off, sound was gone. Went about my business.... Working with tools again later, turned them off and sound was back again...

Couple things, 1, I’m wondering WTH this electronic “running” sound is and 2, I’m wondering if it’s the reason the truck doesn’t spin over as quick as in the past.

Any insights?

Truck is a 2013 TRD Sport 4 door long bed.

 

It's the fuel pump evaporative system.

 

Smog testing. It pressures the gas lines i believe to check for leaks. Its normal. Mine catches me off guard sometimes

 

Damn you guys are quick.

Learn something new everyday.

 

It usually pisses people off that sleep in the bed of the truck when camping.

 

I’ve spent probably 50+ hours standing next to my truck, with it not running, and have never heard this. Odd that it’s a common deal and I’ve never heard it.

 

There is a vacuum pump under the bed that draws vacuum on the fuel tank/canister. Then it sits back and waits to see if the vacuum drops off. If it leaks down then it sets evap codes.

Generally speaking, if the vacuum bleeds down very slowly it will set "evap (small) leak detected".

If it bleeds down a little faster it will set "evap (large) leak detected".

If the there is a wide open hole in the fuel tank (like the gas cap is left off) the vacuum pump will not be able to draw any vacuum at all and it will set "evap (gross) leak detected". Meaning, a leak that is too large to measure..

 

First time I've heard mine today. Started the truck, let it run 10, 15 seconds, no more noise. Around 31,000 on the truck now.
Duane

 

I’ve heard it on mine several times!!

 

If you open the gas cap to accelerate the venting during the sound it immediately stops. My 2015 does it, never caught my 2008 or 2003 doing it.

Usually hear it at odd times if the night when truck has been sitting for hours.

 

When the pump is humming it's not venting, its trying to draw vacuum on the tank and perform a leak test. You're not really hurting anything by taking the cap off but it will prevent it from running/completing its test.

 

That might be the SAI.....

 

What's the SAI? I don't read that many of the tech threads. Many explode with posts.
Duane

 

Secondary Air Injection pump.....

 

Thank you.
Duane

 

I just heard this last night for the first time in ten years on my 2014 long bed Tacoma. I guess I was in the garage at the right time.
Trucks at 100k and never heard this.
Cold yesterday, like 5. Parked in garage and six hours later went to garage and heard humming from truck bed. Thought it was fuel pump? Went on for at least ten minutes. Got keys and with key turn to acc or on noise immediately stopped.
truck started fine. No cels. No lights. Turn off. On again. No cel. ???

 

No, it's the evaporative emissions test as stated above. I've heard my wife's Corolla do it on multiple occasions when I step out into the garage in the evening. I've only ever heard my truck do it a couple of times.
Yes, as you now know, it's normal.

 

So no need to freak out? Working as designed? It was running for like 10 minutes...is that normal?

 

Normal.
Sleep well….

 

Thanks, will do!