I am unfamiliar with the specific features of the Micra brakes but here are some general suggestions. First, does the noise come from the front or the back wheels? If the noise comes from the front and the front brakes are disc the caliper might have lost the anti-rattle spring which prevents the pads from moving when the brakes are not applied. The caliper parts have their smallest dimensions when they are cold but as they warm up after several applications of the brakes the pads may rattle. Get a copy of the parts diagram for the brakes (it appears in the factory shop manuals for the Micra and also in the on-line parts lists for that model from Micra dealers) and check that all parts in the diagram are present on your brakes. Given the age of the car I assume the front pads have been replaced at least once and the mechanic may have forgotten to reinstall those springs. If the noise comes from the rear and/or all brakes are drum, pull the wheel and look for a missing hold down spring.