Depends on the noise.
A squeak usually means metal to metal (lacking lubricant) so can be brake shoes or pads. Cleaning out and lubricating with a little copper grease between shoes and back-plate, or pads and caliper slide faces might cure the noise. Whether I'd go to the bother until new brakes are required I think not, unless there was an effectiveness issue.
A beep is usually regarded as being electronic.
A hiss would normally be air and might indicate a leaking servo.
ChicksFan, November 2011
