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?