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I'm not sure how helpful this will be, but I figured if it can help two or three people then it's a success. There have been a few threads regarding driveline noises and it can be very difficult to explain a sound and where it's coming from. Recently, I was attempting to travel to Corpus Christi to be with family on Christmas day when I heard a small noise over The Whitest Boy Alive's album, Dreams, which I was thoroughly enjoying over my M audio system. Unfortunately or fortunately, depending on which way you look at it, I was just north of San Antonio, the half way point of my trip. I felt it was road debris caught in the rear brake. Being unsure, I pulled over to look the car over. I couldn't find anything loose or dragging on the rear brakes/suspension/wheels and the sound was intermittent and didn't have a noticeable pattern, but I decided I would turn around and drive the 70 miles back home rather than attempt to put over 400 miles total on the car making that noise.

The next week, I took the car over to SoulSpeed here in Austin where it was found that a piece of asphalt had been sitting between the splash guard (heat shield) and the rotor. I wish I had taken a picture of the guard because the way it is shaped is as if there is almost a shelf on the front portion for larger rocks and such to get caught in and stuck until you pull the rotor off or grind the debris into dust. Everything was checked out for possible damage and outside of the rotor having a slight spot on the innermost portion, everything is fine. It's amazing how this thing even got in there in the first place.

Here is a video to assist in diagnosing the sound. I brought up the sound levels and reduced background noise so that you get a better idea of the sound. You'll notice how after a little bit it almost goes quiet then comes back. This is really important in diagnosing drivetrain/wheel & brake noises as some sounds will come and go while others will be cyclical and all may or may not very with speed.

+ YouTube Video
ERROR: If you can see this, then YouTube is down or you don't have Flash installed.
 

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Wow im really glad you posted this. This has happened to me twice!!! At first when it happened, i could barely hear it inside the car, i had the windows down and figured it was the van beside me, untill i turned down my street and realized it was my car. In my case i pulled off the rotor and in the ebrake mechanism i found a small rock that somehow got stuck in there. The other time there was evidence of a rock that had got trapped between the dust shield and the rotor. Was the left rear both times lol. Knock on wood hasnt happened since, so im counting my blessings lol.
 

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I had a small stone between the front disc and its heat shield and it made an almighty screech, even at walking pace! This was at work and it was quite late, as I was the only one around I knew it was my car.
I jacked the car and removed the wheel. The offending stone was small, only about 5mm diameter. A little flexing of the shield had it drop out.
I haven't a clue as to how it got there. Car was silent when I parked up in the morning, screeched as soon as I moved it to go home! At least it was sorted immediately and I had a pleasant drive home.

If you ever get something similar, I reckon it's worth sorting it soon, I'm sure my stone would have scorred the disc in no time.
 
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