What was found
REPEAT WRITE UP. ON APPROACH SELECTED MLG DOWN & A LOUD BANG HEARD & FELT FROM THE NLG. MLG INDICATORS TURNED GREEN WHILE NLG INDICATOR REMAINED YELLOW FOR A PERIOD LONGER THAN USUAL. EVENTUALLY IT TURNED GREEN. ABORTED APPROACH & EXECUTED A GO AROUND & SELECTED MLG UP AS NORMAL. NO ABNORMAL NOISE OR JARRING BANG WAS FELT WHEN THIS WAS DONE. CLIMBED TO ALTITUDE AND DECIDED TO DIVERT BACK TO ORIGIN AIRPORT KDAL. ON SECOND APPROACH WHEN SELECTING MLG DOWN, THERE WAS A SIGNIFICANTLY LESS INTENSE BANG (SEEMED MORE NORMAL) AND INDICATOR FOR NLG TURNED GREEN SOONER THAN FIRST APPROACH. NORMAL LANDING WITH NORMAL BRAKING & TAXING WAS SAFELY COMPLETED. REPLACED VALVE. OPS CHKD GOOD IAW AMM 32-31-00-720-801-A.