My 1997 Olds Cutlass Suprme with 4 wheel disc brake has the ABS light glowing. It makes a clicking noise when the brake is applied when the ABS light is out. Then the light glows and the clicking noise stops. It sounds like it comes from the brake master cylinder or on the firewall. The click is rapid when I'm going highway speed, pulling off an exit ramp. Then the light comes on and it stops. What can it be? Thanks for any info.