I've got the same sound with my Subaru 4Pot brakes on my GT. I asked my local Subaru repair shop (very knowledgable guys). They took it for a spin and looked at it on a lift and told me that it was just the noise of the pad shifting in the caliper under breaking loads. They said it was nothing to be worried about. Apparently it doesn't happen to every subie, which annoys me, but my car stops great, so I have no complaints.