DURING PREFLIGHT; A CHECK AIRMAN NOTICES EXTENSIVE USE OF SPEED TAPE ON THE SEAMS OF THEIR B-777 ALONG THE LEFT FORWARD SIDE OF THE ACFT BELLY. NO RECORD IN LOG BOOK OR DEFERRED ITEMS. ACFT TAPED TO ADDRESS VIBRATION ISSUES.

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DURING PREFLT INSPECTION AS A CHK AIRMAN; I NOTICED EXTENSIVE USE OF SPD TAPE ON THE SEAMS ON THE L FORWARD SIDE OF THE ACFT BELLY. SINCE THERE WERE NO MAINT LOGBOOK ENTRIES; OPEN ITEMS LIST OR DEFERRAL LIST ENTRIES; I CALLED ZZZ MAINT TO DISCUSS THIS ISSUE. I WAS NOTIFIED THAT THIS WAS AN 'X' ISSUE; WHICH IS NOT REFLECTED IN EITHER THE MAINT LOGBOOK OR THE DEFERRAL LIST. I HAVE NEVER HEARD OF AN 'X' MAINT ISSUE; BUT WAS TOLD THAT IT WAS RELATED TO NOISE AND VIBRATION RPTED IN THE VICINITY OF ROW 12 ON THE L SIDE OF THE ACFT. IF AN 'X' IS GOING TO RESULT IN VISIBLE ALTERATIONS WHICH ARE INSPECTABLE BY THE FLT CREW; IT SHOULD BE SOMETHING THAT IS EITHER IN THE MAINT LOG BOOK OR THE DEFERRAL LIST; OR THERE SHOULD BE SOME MENTION OF IT IN FLT MANUAL. I'M CONCERNED THAT SUCH REPAIRS AND ALTERATIONS MIGHT SUBJECT US TO FAA ACTION. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: REPORTER STATED AFTER RAISING THE ISSUE OF SEEING APPROX FIFTY FEET OF SPEED TAPE PLASTERED ON THE LEFT SIDE OF THE FUSELAGE LOWER BELLY; ALONG THE SKIN LAP SEAMS; WITHOUT ANY LOGBOOK OR DEFERRED ITEM; HE WAS INFORMED THE APPLIED TAPE WAS AN APPROVED TEMPORARY REPAIR. FLIGHT ATTENDANTS HAD PREVIOUSLY REPORTED HARMONIC TYPE VIBRATIONS COMING UP THROUGH THE CABIN FLOOR. THE SPEED TAPE IS USED TO VERIFY WHETHER REDUCTION IN VIBRATIONS ARE NOTICED WITH THE TAPE APPLIED. IF THE VIBRATIONS SUBSIDE WITH THE TAPE APPLICATION; THEN MAINT REFERS TO A BOEING MAINT DOCUMENT FOR WING TO BODY FAIRING VIBRATIONS; DIRECTING MAINT TO ADJUST THE VERTICAL GAPS THAT EXIST IN THE WING TO BODY PANELS. ABSENCE OF SUBSTRUCTURES BETWEEN THE BODY PANELS AND FUSELAGE MAY BE A CONTRIBUTING FACTOR. VIBRATIONS SEEM TO BE OCCURRING IN CRUISE AT ALTITUDES BETWEEN 14000 AND 40000 FT. REPORTER STATED THE B777 HE FOUND WITH THE UNDOCUMENTED SPEED TAPE AND VIBRATION ISSUES; HAS BEEN REPAIRED.