B737-300 ACFT IN DSCNT HAD CLRNCS TO STEP DOWN ALTS WITH A XING RESTR GIVEN TOO LATE FOR FLC TO MAKE IT. NORMALLY THE FLC COULD HAVE INCREASED THE DSCNT AND MADE THE XING; BUT THE FLC HAD SELF-IMPOSED A SPD RESTR DUE TO A LOW FREQ VIBRATION IN THE ACFT AT HIGHER SPDS. MAINT SUSPECTED WORN ELEVATOR TRIM BEARINGS.

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OUR FLT RECEIVED DSCNT INSTRUCTIONS FROM ATC TO FL220; THEN EXPECT FL190 AND 280 KTS AT POWDR INTXN. AFTER PASSING THROUGH APPROX FL240; WE WERE ASSIGNED FL200. AS WE CONTINUED DSCNT THROUGH APPROX FL220; WE WERE THEN GIVEN FL190 TO BE MET BY POWDR INTXN; WHEN OUR FLT WAS APPROX 5 NM SW OF POWDR; PROCEEDING TO THE NE. SHORTLY AFTER ACKNOWLEDGING THE ALT RESTR (FL190 AT POWDR); THE CAPT REALIZED IT WOULD NOT BE POSSIBLE TO MAKE THE RESTR; AND I MADE 2 ATTEMPTS ON CTR'S FREQ TO INFORM THEM OF THIS; WITH NO RESPONSE FROM ZDV. AFTER PASSING POWDR AT ABOUT FL198 AND LEVELING AT FL190 APPROX 2 NM BEYOND POWDR; ZDV SWITCHED THE FLT TO DEN APCH. UPON SWITCHING FREQ; THE TCASII ALERTED US TO TFC AT OUR ALT ABOUT 5 NM ENE OF OUR POS. WE RECEIVED NO INFO FROM CTR OR APCH CONCERNING THE TFC; WHICH WAS MANEUVERING AT OUR ASSIGNED ALT; AND UPON CHKING IN WITH APCH; I INFORMED THE CTLR THAT THE TFC AT OUR 1 O'CLOCK POS WAS IN SIGHT. MY CAPT INQUIRED FROM APCH CTL AS TO THE NATURE OF THIS TFC; AS IT APPEARED TO BE OPERATING VFR DUE TO THE RAPID CHANGES IN HDG/FLT PATH IT WAS CONDUCTING. THE CTLR TERSELY REPLIED THAT HE WAS UNABLE TO DISCUSS THE TFC; AND IF WE DESIRED TO INQUIRE FURTHER; WE SHOULD CONTACT HIS SUPVR BY LAND-LINE. AN ADDITIONAL FACTOR AFFECTING OUR FLT WAS A SELF-IMPOSED SPD RESTR OF 300 KIAS; DUE TO A LOW FREQ VIBRATION WE HAD EXPERIENCED FOR MOST OF THE FLT. HAD WE NOT RESTR OURSELVES; IT MAY HAVE BEEN POSSIBLE TO MAKE THE ALT ASSIGNMENT AT POWDR; BUT WE WERE NOT WILLING TO EXCEED SPD 300 KTS DUE TO THE UNKNOWN NATURE OF THE VIBRATION; WHICH MAINT HAD SAID MAY BE DUE TO WORN BEARINGS IN THE ELEVATOR TRIM ASSEMBLY (PROB NOT DISCOVERED UNTIL AIRBORNE). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR FO CONFIRMED ACFT TO BE B737-300. HE IS IN HIS FIRST PROBATIONARY YR AND WAS WORRIED ABOUT THE APCH CTLR RESPONDING TO THEIR INQUIRY ABOUT TCASII TFC. RPTR THINKS THE CTLR WAS UPSET THAT THEY HAD NOT MADE THE ASSIGNED XING RESTR. THE CAPT HAD DECIDED TO LIMIT THE ACFT SPD BECAUSE OF THE LOW FREQ VIBRATION AT HIGHER SPDS. THEY HAD CONTACTED THEIR COMPANY MAINT ABOUT THE VIBRATION AND MAINT HAD SUGGESTED THAT IT PROBABLY WAS DUE TO WORN BEARINGS IN THE ELEVATOR TRIM ASSEMBLY. BECAUSE OF THE SELF-IMPOSED SPD RESTR; THE FLC COULD NOT DSND AS RAPIDLY AS ATC WOULD HAVE LIKED AND THE FLC MISSED A XING RESTR.