B767-200ER CREW HEARD A SERIES OF LOUD BANGING NOISES WHEN STARTING DSCNT INTO ZZZ.

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CARGO SHIFT IN FLT. MOMENTS AFTER BEGINNING INITIAL DSCNT FROM FL410 USING FLCH; WE HEARD AND FELT A LOUD NOISE AND VIBRATION THAT SEEMED TO COME FROM BENEATH THE COCKPIT FLOOR (B767-200ER). THE NOISE WAS SIMILAR TO AN ENG COMPRESSOR STALL; BUT ALL ENG INDICATIONS WERE NORMAL. THE PURSER CALLED IMMEDIATELY AND INQUIRED ABOUT THE NOISE. I ASKED HIM TO CHK THE FORWARD LAVATORIES AND THE CABIN. HE RPTED NO ONE WAS IN THE LAVATORIES AND THAT BUSINESS CLASS GALLEY HAD NOT HEARD ANYTHING; BUT THAT FIRST CLASS HAD HEARD IT. I TOLD THE FO THAT IN 6 YRS ON THE AIRPLANE; I'D NOT HEARD ANYTHING LIKE IT BEFORE. AFTER A SECOND OCCURRENCE OF THE NOISE AND VIBRATION; IN SPITE OF THE FACT THAT THERE WERE NO OBVIOUS PROBS INDICATED ON ANY ACFT SYS OR WITH ACFT HANDLING; I BECAME CONCERNED THAT THERE WAS SOME SORT OF PROB IN THE FORWARD CARGO BAY. WE EVEN DISCUSSED THE POSSIBILITY THAT THERE MAY BE SOME SORT OF SMALL EXPLOSIONS OCCURRING. HOWEVER; THERE WERE NO RESULTANT PRESSURIZATION INDICATIONS. I DECIDED TO DECLARE AN EMER WITH CTR AND REQUESTED AND RECEIVED DIRECT TO THE ARPT. I ASKED THAT THE EMER EQUIP BE IN POS TO INSPECT THE ACFT AFTER LNDG. I BRIEFED THE FLT ATTENDANTS AND TOLD THEM THAT NO EVAC WAS PLANNED BUT TO BE PREPARED. I DID NOT MAKE A PA TO THE PAX BECAUSE I DID NOT WANT TO UNDULY CAUSE ALARM. I THEN TOOK OVER THE PF DUTIES AND COMPLETED AN EXPEDITED APCH AND NORMAL LNDG. WE RECEIVED AT LEAST 3 OTHER OCCURRENCES OF THE NOISE AND VIBRATIONS BEFORE LNDG. AFTER LNDG AND PARKING ON THE TXWY; I MADE A PA EXPLAINING THE SIT WHILE CFR INSPECTED THE ACFT AND FOUND NO DISCREPANCIES. MAINT ALSO CAME OUT AND PINNED THE LNDG GEAR. WE THEN TERMINATED THE EMER AND TAXIED TO THE GATE. I BRIEFED MAINT ON THE PROB AND THEY INSPECTED THE FORWARD CARGO COMPARTMENT. THEY RPTED THAT THERE WERE 4 CONTAINERS PRESENT. 1 CONTAINER WAS SECURED IN THE AFT MOST POS. THE FORWARD 3 CONTAINERS HAD NOT BEEN SECURED PROPERLY AND THERE WAS EVIDENCE THAT THEY HAD INDEED SHIFTED BACK AND FORTH FROM 10-12 FT DURING THE FLT. THE 2 ELECTRICAL SWITCHES THAT CTL THE LATERAL GUIDES THAT LOCK THE CONTAINERS IN PLACE WERE FOUND TO BE IN THE WRONG POS. THERE IS NO UNDERSTATING THE SERIOUSNESS OF THIS HUMAN ERROR. I WAS LATER ADVISED BY SUPVR OF SAFETY DEPT THAT THE 3 CONTAINERS WEIGHED 3500 LBS TOTAL. OUR TKOF PERFORMANCE SETTINGS HAD PLANNED 6119 LBS TOTAL IN THE FORWARD WITH AN OVERALL PLAN OF 13219 LBS AND CTR OF GRAVITY OF 24.9. OUR CLOSEOUT INDICATED A TOTAL CARGO WT OF 13079 LBS WITH A CTR OF GRAVITY OF 25.1. DURING OUR DEBRIEF; THE FO AND I TRIED TO EXAMINE EARLIER INDICATIONS THAT MAY HAVE HELPED UNDERSTAND WHAT WAS HAPPENING AT THE TIME OF THE NOISES. THE TKOF ROLL WAS SMOOTH AND NORMAL. THE FO'S ROTATION WAS A BIT SLOW BUT QUITE NORMAL IN ALL RESPECTS AND WE HEARD AND FELT NOTHING EXTRAORDINARY. SINCE THE FO IS NEW TO THE FLEET; AND DIDN'T HAVE MUCH FLYING EXPERIENCE IN THE B767; WE HAD DISCUSSED EARLIER THE HANDLING DIFFERENCES BTWN THE B757 AND THE B767. HE MADE A COMMENT DURING THE CLB ABOUT THE B767 FEELING 'SQUIRRELLIER' THAN THE B757. AS WE APCHED FL410; EVEN THOUGH THE FMC INDICATED A MAX ALT OF FL421; THE CLB RATE DECREASED TO 200 FPM AS WE WENT THROUGH FL405. LOWERING THE NOSE SLIGHTLY GAVE US 500 FPM AND WE LEVELED AT FL410. WE HAD PROGRAMMED MACH .80 AND HAD SOME DIFFICULTY FOR THE FIRST 15 MINS MAINTAINING MACH WITH A MOMENTARY EXCURSION TO .78 MACH. SINCE THIS IS A CHARACTERISTIC I HAVE EXPERIENCED MANY TIMES; IT DID NOT THEN CAUSE ANY ALARM. HOWEVER; THE PWR TO MAINTAIN MACH .80 APCHED MAX CRUISE SETTING INITIALLY. THE REMAINDER OF THE FLT UNTIL DSCNT WAS UNEVENTFUL. I WOULD LIKE TO KNOW THE EFFECT OF 3500 LBS SHIFTING FROM THE FORWARD TO THE AFT SECTION OF THE CARGO COMPARTMENT. IN THAT THIS IS CLRLY A HUMAN FACTOR ERROR; THERE MUST BE SOME MECHANISM PUT IN PLACE THAT WILL PREVENT A FUTURE RECURRENCE. EVERYTHING IN MY JOB AS A PLT ENTAILS THE USE OF CHKLISTS AND XCHKS. IT SEEMS LOGICAL THAT AN ERROR OF THIS NATURE; THAT HAS SUCH A CATASTROPHIC POTENTIAL; WOULD EASILY BE PREVENTED BY AT LEAST 2 PEOPLE INSURING THAT THE CARGO IS PROPERLY SECURED. IN THAT THE AVIATION INDUSTRY HAS IN RECENT YRS HAD SEVERAL FATAL ACCIDENTS THAT WERE CAUSED BY IMPROPER CTR OF GRAVITY COMPUTATIONS AND CARGO SHIFTING; WE CAN'T AFFORD THIS KIND OF ERROR.