On my 1977 Spider I have a buzzer that periodically goes on when the brake pedal is not depressed, however it is deactivated if the pedal is even just tapped. I have not been able to identify it and when I start to focus on the problem it just disappears.
Look up under the dash at the steering column on the right side.
You'll likely find 2 small gray boxes with 2 wires going to each. Those are the buzzers.
Check around that area and the brake pedal, particulalry how the wires in that vicinity are laid out, as I imagine you've got some sort of short going on that occurs when the brake pedal pinches or pulls the wrong thing.