Check the slides in the caplipers they may be loose or have some play in them enough to rattle. Especially if the rattle stops when a little pressure is applied to the brakes. If there is play, it will be apparent. Good luck and let us know what you find out.

__________________
1990 190E 2.6
1996 Grand Voyager 3.3
1985 Mustang GT 5.0 5 SPD
1982 Suzuki GS 750T