98 ABS light trouble

For a while now, the ABS light has been consistently on. First it started turning on after driving a short amount of time, then it started turning on and staying on after starting the car and waiting a minute or two, and now every time the car is started, the ABS light stays on right away. I noticed now that when I turning the ignition to "on", right away I can hear a quiet "click click" which results in the light staying on and not turning off (it does this all the time now). Is there a common problem to this issue or is it more specific?

1) Check your Brake fluid.
2) Do a Brake inspection.
3) Clean all four wheel speed sensors with brake cleaner.

All of the above...Most common cause is related to wheel sensors, specially when front hub bearing go haywire..you would hear a humming sound. An advanced code reader will reveal the source of the ABS light being on, which wheel is the culprit, or ABS internals. Corroded contact at wheel sensor connectors happen to pop the light.
Low brake fluid would illuminate the red brake light, not ABS...but check level anyway.