A GULFSTREAM II CLBING THROUGH FL200 NOTED VIBRATION IN FLT CTLS. ALL COCKPIT INDICATIONS INCLUDING TURBINE VIBRATION NORMAL. AT LEVEL OFF AT FL280 VIBRATION STOPPED. ON GND FOUND 30 PERCENT OF LEFT NOISE SUPPRESSOR MISSING.

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ON 10/WED/05; AT APPROX XA00 LCL; I DEPARTED TEB ARPT AS THE CAPT OF A GULFSTREAM II CORP JET. THIS PARTICULAR ACFT IS EQUIPPED WITH AFTER-MARKET ENGINE NOISE SUPPRESSORS. THESE UNITS; MADE BY REALLY QUIET LLC; ARE 48 INCH LONG BY 30 INCH DIAMETER ALUMINUM TUBES THAT ARE AFFIXED AFT OF THE ENGINE TAILPIPES. THE WEATHER WAS CLR AND THE TKOF WAS NORMAL. APPROX TEN MINS INTO THE FLT; WHILE CLBING THROUGH 20000 FT; AN ABNORMAL VIBRATION WAS FELT IN THE CTLS OF THE ACFT. ALL COCKPIT INDICATIONS SHOWED NORMAL AS WELL AS THE ENGINE GAUGES AND THE TURBINE VIBRATION INDICATOR. THIS INDICATOR REGISTERS ANY ABNORMAL VIBRATIONS IN THE ENGINES. I LEVELED THE PLANE OFF AT 28000 FT AND DISCUSSED WITH MY FO THE POSSIBILITY OF MAKING A PRECAUTIONARY LNDG. WITHIN APPROX 5 MINS; THE VIBRATION ABRUPTLY STOPPED. AFTER 10 MINS OF CHKING EVERY POSSIBLE CAUSE OF THE ABNORMALITY; AND CONCLUDING THAT NO HAZARD STILL EXISTED; I ELECTED TO CONTINUE THE FLT. AFTER SIX HRS; WE LANDED UNEVENTFULLY AT LAX ARPT AT XD00 LCL TIME AND DE-PLANED OUR PAX. ON THE POSTFLT WALK-AROUND; I NOTICED THAT APPROX 30 PERCENT OF THE NUMBER ONE ENGINE'S NOISE SUPPRESSOR WAS MISSING. APPARENTLY ITS DISINTEGRATION DURING THE FLT HAD CAUSED THE TEMPORARY VIBRATION WE HAD FELT. THE FAA WAS NOTIFIED AND ISSUED MY COMPANY A FERRY PERMIT TO FLY THE PLANE BACK TO OUR HOME BASE WHERE IT IS BEING REPAIRED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE NOISE SUPPRESSORS WERE AFTER-MARKET UNITS RECENTLY INSTALLED AND AS FAR AS HE KNOWS; WERE FITTED TO THREE AIRPLANES AT MOST. HE UNITS ALLOWED THE AIRPLANE TO MEET STAGE 3 NOISE LIMITS. THE AIRPLANE IS STILL IN MAINT AND THE CAUSE OF THE L SUPPRESSOR TUBE FAILURE IS STILL UNKNOWN. THE COMPANY THAT HAD THE SUPPLEMENTAL TYPE CERTIFICATE HAS GONE OUT OF BUSINESS AND SOLD THE SUPPLEMENTAL TYPE CERTIFICATE TO ANOTHER PARTY. THE ADDITION OF THESE NOISE SUPPRESSORS IS A BIG JOB THAT REQUIRES A LOT OF SYSTEMS MODIFICATION INCLUDING WT AND BAL PROBS. CALLBACK CONVERSATION WITH RPTR ACN 676343: THE RPTR STATED A FEW MINS AFTER REACHING FL280 THE VIBRATION STOPPED. THE FLT CTLR VIBRATIONS WERE MODERATE BUT NOTICEABLE. GND POSTFLT INSPECTION REVEALED 30 PERCENT OF THE L SUPPRESSOR TUBE MISSING. THE SUPPRESSOR SYSTEM IS ABOUT TWO YEARS OLD AND ONLY INSTALLED ON A FEW AIRPLANES.