weird noise question
 

Hey everyone,I'm new here and this is my first post. I currently own a 2008 Mazda CX-9. This is my first Mazda and love it. It is a little over a year old and I recently started to hear a noise coming from the front left side only when I brake. It doesn't happen all the time.
During a recent service(oil change) I brought it up to the person at the service department. When I explained the problem one of the mechanics claimed to know exactly what I was talking about and said it was common. He said it was something in the braking system where dust or ohter foreign matter gets between this so called plate that causes that. He said it would happen if it was cleaned out. I have a feeling they were giving me the runaround. Does anybody have this same problem?
The car has never been hit or hit any hard potholes or curbs. I am the only one who drives this car so I know for sure.
Another thing i noticed is that the bolts holding the doors on on the hinges in the door jam on all four doors have started to rust. I also addressed this problem but was told to call back which I will do so tomorrow. Has this happened to any one else?
Thanks in advance