Turbo-like air/electronic sound observed during idle and drive

Howdy folks, I am curious about whether I can get some insight into what this could be and if anybody has ever heard this before. I have a 2011 Tacoma TRD Off Road 4x4 with the electronic brake unit and the 5sp auto transmission with the 4.0L V6.
Once (maybe twice) a year, I sometimes will hear a sound that sounds like part air/part electronic - like on the driver side inside of the engine bay near the fuse box and the "special" braking contraption. It is minimal and barley noticeable inside the cab of the truck if you're listening for it. There are no lights/warnings/codes, and does affect performance in any way, it's just the noise that I might hear, but once I shut the truck off and start it again, it's gone like it just disappeared. The sound does change when increasing/letting off the gas, but doesn't wine like a turbo - sounds similar to a turbo it in a way.
I have checked for vacuum leaks, and looseness on connection points and have not discovered any issues. I haven't heard the sound for days now, but am skeptical of whether it's a test of the vacuum lines/booster/fan/alternator/air compressor, or is occasionally something that should be examined more closely. Due to it being random, and so sparsely noticeable, it's a mystery to me. Everything under the hood seems to be in good shape, it's obvious to hear directly above the driver-side suspension strut.

Any ideas/help is greatly appreciated, thanks!

 

Sounds like the SAIS pump. I believe it's on 2011-2015 trucks.

Also could be the brake booster building up pressure. It's just an electric pump and an accumulator.

the SAIS you usually you hear them during startup and shutdown? maybe just shutdown? It definitely sounds like a turbo spinning down if you are not familiar with it.

 

Gotcha, its not really noticeable to shutdown and startup, it is sustained while driving and the noise changes with the gas pedal... Would the SAIS pump still do that? I don't think the booster would either, but a tech told me it could maybe be ABS turning on randomly, could that be a possibility as well?

 

During driving for long periods should not be either of my guesses unless they are significantly broken.

Does it change with pedal movement, rpm, or speed?

If you recently had some steering work done, like parts replaced or an alignment it could be the zero point setting. That would kick on the ABS pump because it thinks its slipping. You should feel that in the brake pedal and it should cut power and give you a flashing traction control light though.

I'd not rule out a simple exhaust leak either. Those can make some weird noises

 

It does change with pedal movement, and speed, but no codes or lights are thrown and performance remains steady. I did just recently have steering work done (outer tie rods, upper arms, and an alignment), but I have heard this long before that work was done. This is only the 2nd time if ever heard it in the last year. Brakes sound/look/feel great, and no trac control gets thrown on the dash...

Could be an exhaust leak, but 99% of driving, everything is just fine, and I can't hear or notice the noise.
I'll probably sell the truck anyways next month, but was curious if maybe I could solve the mystery lol

 

Guessing it’s not alternator noise like heavy demand. Just a thought…

 

I'm out of ideas. OP, can you post a photo of the engine bay?
(secretly hoping to see a SC in there)

 

Sure, here are some photos of the bay as well as on both sides...

 

no SAIS pump, I guess yours was not equipped.
Electric brake booster, so thats not a source of a vacuum leak
Evap can should be behind the cab under the bed, so thats unlikely to be causing noises.

I'm still stumped.

 

So am I man, so am I lol

 

Does the sound change with engine temperature?

 

I'm not really sure, the sound goes up like a small turbo when I press on the gas and then goes back down when I let off the gas (similar to Ford Transit van). I don't think I've seen it be dependent on temp although both times I've heard it, it occurs on a cold day with a cold engine after having been shut off and sat for 4-6 hours.

 

Tell me it’s not coming thru the radio speakers. If that’s happening you want a filter on the radio power supply line. You’re sayin’ you heard it outside but not inside the cab, right?

 

Yes, I heard it outside the cab right above the drivers side front suspension strut and right next to the fuse box area, but can barely notice it inside the cab. Ac/heat is off and the music was off as well.

 

Maybe try popping the cover off the fuse/relay box & see if everything is in place. Could be a relay buzzing but other than that I’m out of ideas.

 

try this next time its cold.

turn key to on (do not start), press brake pedal multiple times.

This will kick on the brake booster after a few presses. You should not hear the pump for a long period, just a second as it builds pressure again. If you hear it run for a long time then you might need to replace that expensive beast. (I am looking into this on my truck)

 

Here's the thing, the electric brake, trac, abs all-in-one unit is brand new (as of 2 years now)...
That's part of the reason why I may sell the truck, I want one that's not so sophisticated like this one - one that drives and acts like a car, but have 4WD lol

 

Whoa... then it's unlikely to have failed. Still worth testing to see if thats the source of the noise after exercising it.

 

I can certainly give it a try next time I drive, but the weird part is that I haven't heard it since Saturday, and the brakes and everything feel great. To replace that brake assembly was like 1,500 bucks at the dealer, and part of it was because they have a specific programmer/scan tool made just for that part. Maybe it is exhaust related, but because it was mostly on the drivers side, I feel that may be unlikely. Could be a fuse relay thing like FastEddy59 said, but because its so sparse and so random, I'm unsure what it could be lol

 

Mine just started sounding like a jet engine. The only rotating mechanical components I haven’t changed on the front of the motor are the compressor and alternator. So I am going to be replacing them. The truck is old and has miles. There’s nothing to think about.