What was found
ON FLIGHT 4236 (CMH - MSP) A LOUD CLACKING NOISE AFTER TAKEOFF. WHEN GEAR WAS SELECTED UP A LOUD REPETITIVE GRINDING/THUMPING NOISE WAS HEARD FROM THE NLG AREA. THE NLG SHOWED IN TRANSIT AND MLG SHOWED UP. THE NLG DIDN'T SHOW UP AND LOCKED UNTIL 6 MINUTES AFTER THEY WERE SELECTED UP. TROUBLESHOOT CLACKING NOISE BY SERVICING NLG STRUT IAW AMM 12-12-32. REMOVED AND REPLACED NLG UPLOCK ASSY IAW AMM 32-33-05. RIG AND OPS CHECKED NLG RETRACTION AND EXTENSION UPLOCK. OPS CHECK NO DEFECTS NOTED AND OK FOR SERVICE. SERVICED NLG WITH NEW OIL AND NITROGEN IAW CRJ AMM 12-12-32. AIRCRAFT WAS ON GROUND AT TIME OF SERVICING.