I think its the pad itself,new discs are worn in apart from a half inch ring in the middle of the disc,same on both sides,done it to my previous grooved discs aswell,think the pad isnt biting on the centre of the disc!has to be pads,shuda changed them anyway.the noise comes and goes...I only got the pads at christmas,after bout a week i noticed a black kinda friction ring in the centre of my disc like something was stopping it from goin the whole way,maby just dodgy pads.