If you didn't put the retaining clips in properly (or left them out), the pads will rattle ALL THE TIME except when your foot is on the brake pedal applying pressure- sounds like a can full of rocks with the windows down. Ask me how I know.

 

It's the sunroof

The second day i had my car I heard that noise and for no apperent reason i reached for the sunroof and grabbed that metal piece. That was that

 

I had the same sound when I had my 04 530i I just ignored it now a yr later I have a 07 550i and every once and a while I hear that sound so I took my hand and pressed on the headliner where the sunroof buttons are and boom the sound when away so it's up there somewhere and they are lightly tapping like 2 hollow pipes. Haven't heard it lately

 

There is a metal plate/deflector that can rattle very loudly. just need to grab it in the center with your hands and bend the middle down a bit.....had the same problem

 

Well. .i think you guys solved it! Rattle came back the other day so I pulled back the sunroof cover and pushed that metal deflector and it was the noise! Glad I know it's the source and how to fix it! Thanks! !

 

I first noticed that rattle when I was test driving my car. I figured the previous owner had a couple dinner plates stacked under the seat or something. I ended up sticking a strip of velcro (just the soft side) to the top of the metal deflector and it hasn't made a sound since.