ACFT EQUIP PROB. EXCESSIVE ENG VIBRATION AT ALT IN ICING CONDITIONS. ACFT MD90 WITH IAE2500 ENGS. ENG ANTI-ICE HEAT ONLY TO COWL. BULLET NOSE IS MADE OF RUBBER AND THAT BEING NOT SYMMETRICAL -- VIBRATES; THEORETICALLY TO SHED ICE.

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IN CRUISE FL310; IN CIRRUS LAYER WITH ENG AND WING ANTI- ICE ON WITH LIGHT STRUCTURAL ICING '#2 ENG VIBRATION HIGH' LIGHT CAME ON. #2 VIBRATION GAUGE SHOWED 4.5-4.8 (4.0 BEING THE LIMIT). COMPLIED WITH CHKLIST AND PULLED PWR BACK TO IDLE. VIBES GUAGE STILL SHOWED 3.5-4.5 RANGE. COMMENCED DRIFT DOWN; REACHED VMC BY FL280. VMC CONDITIONS DID NOT REDUCE VIBE INDICATIONS. DIVERTED TO LUBBOCK; TX; FOR LNDG. VIBES REDUCED TO NORMAL AROUND 12000 FT. BROUGHT #2 UP FOR NORMAL 2 ENG LNDG LBB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR FO IDENTED ACFT AS MD90 WITH IAE V2500 ENGS. THE ACFT WAS IN ICING CONDITIONS AT THE TIME OF EXCESSIVE VIBRATIONS. FLC FOLLOWED PROCS AND PULLED PWR BACK TO IDLE; BUT THE VIBRATIONS WERE STILL OUTSIDE OF LIMITS UNTIL THEY DIVERTED AND DSNDED OUT OF ICING CONDITIONS. RPTR HAS HAD NO FEEDBACK FROM HIS COMPANY; BUT MAINT JUST PLACARDED THE VIBRATION INDICATORS AS INOP (MAINT CARRYOVER ITEM -- MCO). RPTR THINKS THAT THE RUBBER BULLET NOSE WAS THE SOURCE OF THE EXCESSIVE VIBRATION WHEN ICE WAS BUILDING UP ON IT. THOUGH; THE VIBRATION FROM THE UNSYMMETRICAL SHAPE IS DESIGNED TO SHED THE ICE AS IT BUILDS UP. HOWEVER; RPTR THINKS THERE WAS AN ICE ACCUMULATION BEFORE IT WAS SHED RESULTING IN THE VIBRATION. THEY DECLARED AN EMER AND DIVERTED BECAUSE THEY THOUGHT THEY HAD NO USE OF THE ENG SINCE IT STILL HAD HIGH VIBRATION AT IDLE PWR; UNTIL AT LOWER ALT.