What was found
AIRCRAFT WAS GROUNDED: F/A ADVISES LOUD BANGING COMING FROM UNDER THE WING. METAL ON METAL SOUND WHEN BRAKES APPLIED WHETHER TAXING OR NOT. SOUND LOUD ENOUGH TO ALARM PAX AND FLIGHT ATTENDANTS. MAINTENANCE DUPLICATED NOISE IN MID CABIN AT L3/R3 DOOR AREA. IAW TSM 32-42-00-810-841-A, JACKED MLG - CHECKED ALL 4 TIRES/BRAKES FOR PRECAUTIONARY. REMOVED & REPLACED AUTOMATIC SELECTOR 2576GM IAW AMM 32-42-13-400-001-A. ACCOMPLISHED SATASFACTORY OP'S CHECK OF NORMAL & ALT BRAKING SYSTEM. NO LEAKS NOTED.