just to help any one whos had same problem
now im to sure if noise was from rear or front but when your in reverse and touch the brakes you get a clank noise now i had a rac inspector look at it and said to take it to lexus now they have finally fixed the problem after having the car back 3 times
what the problem is, is a caliper spring clip that is either broke or worn therefore not relesing the brakes properly therefore getting the clank when it finally releases
hope this helps anyone else whos got the same problem or gets this problem