The noise only occurred when the car was moving, and did not intensify when I braked. I couldn't tell which wheel it was coming from.

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The car in question is a 2012 VW Polo. I had last driven it 4 days earlier on country A-roads for about 75 miles there and 75 miles back. Then today after I left the house and had driven about 100m on the lane at about 20-25mph, I suddenly started hearing a high-pitched scraping noise.

I drove less than 100m more and the noise disappeared, then came back after another ~100m when I got to the end of the lane. As I joined the slightly faster main road, the noise disappeared again, this time for the whole of my 12 mile round trip.

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I got out and determined there was nothing hanging off the bottom, and nothing I could see in the wheels. The scraping sounded very similar to the noise in this YouTube video.

Is there any chance the emergency brake is slightly engaged? I would suggest jacking the wheels of the ground and spinning them to see if you can pinpoint the sound. It may be something stuck next to the brake pad (which wouldn't necessarily get louder when the brakes are applied), or it could be a sticky brake pad that is near the end of its life.

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If you have access to a infrared digital thermometer, you could go for a drive and wait for the sound to occur for a few minutes, then get out and check the temperatures of the brake discs. If one of the discs is significantly hotter than the rest, then you likely have a sticking brake pad on that wheel.

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