Buzzing sound from the front with engine off

This one has me puzzled.

Tonight I had to bleed the brakes and check out my front parking lamps (they were dead)
I went ahead and used on the brakes. I don't think I did a great job, because I was getting lots of air. I made sure that the reservoir never went down below halfway.
Went for a short test drive and came back, parked in garage.
I turned off the truck, then turned it back to the ON position
I got out of the truck and noticed an electric hum from the front end. It's a pretty low sound, probably not much louder than the fan in my computer.
If I press the brakes, a louder humming comes from the rear, and the front continues humming.
If I let off the brakes, the front keeps humming.
If I turn the truck off, the humming immediately goes away.

During my test drive, I made sure the ABS worked
I have an ABS kill switch (makes the truck think the diff lock is on), and toggling that doesn't effect it
All lights are off.
I don't have VSC

What gives? It sounds like it's somewhere underneath the engine, but too hard to pinpoint.

 

Brake booster, normally makes a slight noise, you may have air in it which is making it louder.

 

I was expecting it to stop after a short bit, but I tested and it was still going after 2 minutes. I'm guessing I just had tons of air in the line. decided to have a shop bleed it because I was done with farting around with it.

 

Trying to figure this one out. The booster works on vacuum, so yeah, there will be air in the booster on one side as the pedal is being pressed. Are you trying to say that he has a vacuum leak on the supply side?

 

ABS pump?

 

shop found no leaks. they took the air out and found a bad wheel bearing (unrelated). I didn't ask about the humming yet because I wanted to eliminate some of the easy stuff first. I haven't had a chance to check the noise again to see if its still there.