What was found
AT FL 330, RIGHT BEFORE DESCENT, STRONG THUMPING FELT IN FLOOR. IT FELT LIKE IT WAS NEAR THE FLIGHT DECK. THIS LASTED APPROX 10 MIN. AFTER THAT THE REST OF THE DESCENT WAS NORMAL UNTIL PASSING 10,000 FT. THUMPING IN FLOOR RESTARTED FOR SEVERAL MIN. BOTH F/A'S SAID THAT THEY COULD HEAR AND FEEL THE THUMPING ABOUT AT ROW 4. REMOVED FROM SERVICE. PERFORMED RIGGING AND FUNCTIONAL TEST OF NLG UPLOCK 2 SENSOR AND REPLACEMENT OF SENSOR. WIRING CHECKS PSEM2 TO NLG UPLOCK2 SENSOR PLUG, AND REPLACEMENT OF PSEM2. OPERATIONAL CHECK FLIGHT FAILED. AIRCRAFT REMAINS OUT OF SERVICE.