1995 E320 Here's one I've never experienced before...

I just noticed that when I depress the brake pedal (I'm talking within the first centimeter of the pedal being depressed) I hear a very brief buzz that lasts a half-second, then disappears. I reapply the brake pedal, it does it again. I think the buzz is coming from behind the instrument cluster (?) The sound is so fleeting, it's hard to tell. What is this?