2005 Ford Escape brake indicator accompanied by a sound. What is wrong?

I have a 2005 Ford Escape and when I get in the car. I turn it on and the sound to put my seatbelt comes on then it turns off and when I begin to drive it starts again. The same sound. The brake light indicator is also on. I know the sound has to do with the brakes because when I come to a stop light or something then the sound stops or if I'm driving for more than 20 minutes then it'll stop. It will literally make that ding, ding, ding sound over and over for 20 minutes. I have no idea what it is! The brake fluid is fine so it's not that.

What you are supposed to do is put on the safety belt BEFORE staring the engine! Try that