I have a 2006 automatic Mustang GT that is making sounds when I drive at slow speeds on the left side of the car. I can't tell if it is coming from the front or rear. I have had the front and rear brakes checked out and it is not them. Once I hit over 40 miles an hour or put my brakes on, it stops. It started out sounding like a ch ch ch ch sound every once in a while now it happens all the time. Any thoughts? Help please!

 

Brake dust covers. On the back of the rotors.

99% of the time thats what it is from what I've seen.