Mine does the same. I don't know exactly what it is but it doesn't seem like anything is wrong. I think it is the sounds of the pads on the profile of the disc. It may be the compound of the pads or the pattern/cooling vents on the disc. Are you running standard pads and discs?
Mine does it too..Normal..Though on the 'Northern Lads Rideout' there was also a sound of squealing, and a few oh god oh god oh god from the bike in front..Could have been coming from the brakes, or possibly the rider heading for the stone wall..