Brake Click

I noticed this morning that when I turn on the ignition on my '97 4.6 that the brake Pedal gives a solid click. It doesn't click when I press the press the pedal so I assuming that it is not the well documented brake modulator fault. Also (high risk statement coming up) my brakes are fantastic.
Anyone got any ideas?

The pedal on mine clicked when i bought it. It did so for 10 months, but stopped after i freed the stuck caliper guide slider on one of the calipers. Brakes feel totally different (softer) now, but the power is all there.
Check the sliders, pads and discs before you spend cash on other things.
Good luck

Maybe something to do with the brake servo getting a vacuum as you start the engine and allowing the pedal to move slightly ?