High pitch electrical tone when pressing brake pedal...

Hi all,
Had my Tacoma for around 25,000 miles now and just began to notice a very high pitched, constant, electrical tone whenever I press the brake pedal. Easiest way to hear it is to press the brake pedal with the truck off...definitely noticing it every time I touch the pedal now and I never heard it before.

Anyone else ever notice this? It's really quite annoying, having the fan or radio on will drown it out, but I'm wondering if it's common or I'm experiencing something unusual.

 

No I've not noticed it. Could it be a relay buzzing? Do "american" cars use brake light relays? I'm more used to the mercedes monster relays.. don't deal in dash boards too much.

 

could it be the electric motor that helps the brake booster with vaccum, or possible a teeny vac leak?, maybe even the brake light switch?