What is this buzzing noise in ignition mode (engine off)?

I noticed a strange, continuous buzzing noise when in ignition / accessory mode (two start button presses without your foot on the brake). No fan or anything else electrical on in the cabin.

I’ve been searching to see what this might be and I found the below YouTube video that demonstrates what I’m hearing:



I recently swapped my speakers out with some components up front and they seem more sensitive to whatever this electrical noise is, and it can be heard through them when the car is in ignition mode or with the engine running.

 

That's the brake booster accumulator pump building pressure. It's supposed to do that.

 

That would be the sound right at the beginning of the video, correct? Would the continuous “electrical transformer’ sound be also from the same pump? That’s what I’m wondering about.

 

Ah. I didn't have the phone turned up loud enough to hear that the first time.

I don't know what that is. I've never heard it from my truck, but that doesn't necessarily mean my truck doesn't make the same sound. I'll listen for it and let you know.

 

Here’s a video with the noise from my truck (may need to turn volume up):

 

Dealer replied back and said they weren't sure, "it's normal" ... any ideas?