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My car is a 2004 Subaru Impreza WRX with 72k miles. About three weeks ago I started noticing a scraping sound when I turn left. My mechanic did a courtesy check of alignment, balance and tire pressure about two weeks ago, but didn't find anything that could've caused the sound.

Here's where I am with the diagnosis right now: The sound is obvious when I turn left at speed, but inaudible when I turn left slowly. After turning left fast, if I then straighten the wheel, the sound goes away, but not immediately – the intensity of the sound is not related to how hard I turn the wheel left, but seems to be in how much the car tilts on its springs as it turns. (Once or twice I believe I've heard it while going straight, hitting bumps that cause the car to lean slightly left, but I have a hard time reproducing that with any regularity.)

The power steering fluid level is OK. The sound is not associated with any handling problems; it clearly comes from the rear of the car, and at first I thought something was scraping on the ground. There's definitely nothing hitting the ground while the car is stopped, and it's difficult to look carefully under a moving car while it turns left fast.

How can I diagnose what is causing the sound?

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up vote 4 down vote accepted

Three possibilities occur to me right away:

  1. Nick could be right in this other answer: it could be a CV joint.

  2. Warped / high spot on the break rotor: this happened on my old Ford product, way back in the day. The brake rotor was ever so slightly off kilter that, when I would turn the wheel slightly left, the pad would touch the rotor lightly. Result: a very embarassing squeal / scrape.

  3. Slightly loose lugnuts: I have a car similar to yours and had a similar sound after rotating my tires some months back. It was easy to jack that side of the car up and ensure that the nuts were torqued to the right spec. I put the car back down and the noise was gone.

With your car, I would start with point 3. It's easy and might solve the issue in 15 minutes. If it doesn't, you haven't made your car worse.

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Turns out the problem was #2, although I didn't find it all that embarrassing. :) –  kojiro Dec 30 '12 at 21:56
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