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It will be the Brake wear indicator. It's telling you that the brakes need to be replaced soon .... not as in straight away, but in the next few months or weeks (depends on how hard you brake). I normally replace them when they are a constant loud screech.
The other thing is that it could be a build up of brake dust, get a power hose and blast the wheels ... that should remove any build up .... be careful though ... as your brakes will be wet and braking wont be as responsive.
Lastly ... it could be because of the brakes over heating and expanding causing the shims to hit the brake disc ... the recent hot weather has caused a number of people to think they need to change their brakes, when it's just been the brake expanding due to the heat.
I wouldn't worry too much about it .... if your really conserned ... pop into your local garage, and they should be able to tell you if the brake pads need replaced yet.