What was found
PDX - AFTER TAKEOFF OUT OF GEG, GEAR CAME UP NORMALLY, BUT THERE APPEARED TO BE EXCESSIVE NOISE AND VIBRATION. "UP" NOSE GEAR INDICATION OCCURRED ABOUT 2 SECONDS LATER THAN THE MAINS. ON APPROACH, GEAR EXTENDED NORMALLY. INTERROGATED PSEU, NO FAULTS FOUND. FOUND FLAT SPOT ON NLG TIRE, REMOVED AND REPLACED BOTH NLG TIRE ASSY IAW MTC 6324102. GREASED NLG COMPLETELY, C/W A FULL FUNCTIONAL TEST OF THE LANDING GEAR EXTENSION AND RETRACTION IAW Q400 AMM TASK 32-31-00-720-801, MAIN UP AND LOCKED WITHIN 5 SEC, NLG UP AND LOCKED WITHIN 7 SEC, NO DEFECTS NOTED, OK FOR CONTINUED SERVICE.