2000 Volvo S80

My wife's 2000 2.9 S80 (80k mi) has a high pitch ringing noise which occurs intermitantly. The noise goes away when the brakes are appied. I do not believe this is caliper / rotor related since the noise can occur when the car is stationary. Is there anything in the ABS which may be causing this problem?
March 25, 2007.

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