ENG VIBRATION AND SHUTDOWN INFLT.

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FLT AB FROM ATL TO DTW; I WAS 1 OF 2 EXTRA CREW MEMBERS COCKPIT JUMPSEAT RIDERS. AFTER IDENTING MYSELF TO THE CREW ON THE GND BEFORE DEP; THE CAPT EXTENDED ME THE OPTION OF TAKING A SEAT IN THE CABIN. I SAT IN ROW 28 NEAR THE R HAND EMER EXIT DOOR; NEXT TO ANOTHER COMMUTING PLT. AT ABOUT XX45 UTC WE HEARD A NOISE FROM ONE OF THE ENGS APPARENTLY; AND FELT A SLIGHT VIBRATION. WE SUSPECTED AN ENG MALFUNCTION. THE CABIN CREW INTERRUPTED THE BEVERAGE SVC; AND THEREAFTER A PUBLIC ANNOUNCEMENT FROM THE COCKPIT CREW NOTIFIED THE PAX THAT THE CHKLIST WAS ACCOMPLISHED FOR A MALFUNCTIONING #2 ENG; AND WE WERE PROCEEDING TO DETROIT WITH THE #2 ENG AT IDLE PWR. THE VIBRATION SEEMED TO INCREASE AFTER SEVERAL MINS; AND I NOTIFIED A FLT ATTENDANT TO TELL THE COCKPIT CREW OF THE VIBRATION; AND SUGGESTED A CREW MEMBER TO COME BACK TO CHK IT. THE SO CAME BACK AND INFORMED US OF THE PLAN TO IDLE THE ENG AND FLY TO DETROIT THAT THE ENG PARAMETERS WERE WITHIN LIMITS AT IDLE. LATER IN THE FLT I ENTERED THE COCKPIT TO OFFER ANY ASSISTANCE; BUT NONE WAS SOLICITED. I SAID I WOULD BE SEATED IN THE AFT OF THE PLANE TO MONITOR THE VIBRATIONS. AS AN EXTRA CREW MEMBER I FELT SOME NEED TO BE OF USE TO THE CREW. THE #2 ENG WAS SHUT DOWN BEFORE LNDG. I INFORMED A FLT ATTENDANT THAT IF AN EVACUATION WAS NECESSARY; I WOULD ASSIST OUTSIDE THE AIRPLANE AT ONE OF THE AFT EMER EXITS.