1988 Jaguar XJ6 Anti lock failure

Sounds like something may have broke or popped loose.Did it lose any brake fluid on the ground? is the master cylinder resevoir full? May have to remove thetires and visually look to be sure it isnt anything obvious like a pad that fell out from being worn to a brake hose that has begun to leak

No fluid loss,resevoir is 3/16 below filter. Anti lock message disapeared, low brake pressure message remained. Was backing up again same noise anti lock message returned. No problem or noise when braking. Removed tires no problems. Is there any thing in the Power Booster housing that has any thing to do with Anti Lock. Any thing I should know before inspecting P/B

So it only does this when backing up?

So Far yes

I would try spinning the wheels backwards while inspecting the brakes to see if something is loose or if a bolt fell out.If it only does it in reverse, it must be something mechanical not right, if it was s sensor or the such it wouldnt care which direction the car was moving