Your ABS does a "self test" for the first 20-30ft everytime you start off right after a start up. (forward or reverse) Early E34s had a glitch (really rare) that it would not complete the cycle. Techs I know did one of two things to cure it. #1) Restart and drive about 30 ft then hammer the brakes. If that doesn't cure it, disconect battery for1minute then reconnect and drive about a mile at normal speeds & braking. If these don't cure it ,check out the wiring connectors/sensors at the wheels - you might have one sticking.