A B757-200 DURING FLAP RETRACTION HAD AIRFRAME VIBRATION. RETURNED TO DEP ARPT. FOUND L PRECOOLER DOOR BRACKET BROKEN. DEPARTED A SECOND TIME EXPERIENCED SAME VIBRATION. ON APCH HAD FALSE GND PROX WARNING. RETURNED TO DEP ARPT.

Narrative

DURING FLAP RETRACTION AFTER TKOF FROM ZZZ; THE AIRFRAME BEGAN TO VIBRATE. AS WE ACCELERATED; THE VIBRATION BECAME WORSE. CAPT AND I DECIDED TO RETURN TO ZZZ. AS FLAPS WERE EXTENDED; VIBRATION STOPPED. ON RETURN; I NOTICED A MAP SHIFT ON EHSI. DIDN'T THINK TOO MUCH ABOUT IT AT THE TIME. WE LANDED AT 202.0 LBS WHICH IS AN OVERWT LNDG. LNDG WAS SMOOTH AND UNEVENTFUL. WE NOTIFIED MAINT AND THEY BEGAN THEIR SEARCH FOR A CAUSE. HAVING REPAIRED WHAT MAINT THOUGHT WAS THE CAUSE (A BROKEN BRACKET ON THE L PRECOOLER OUTLET DOOR); WE RE-BOARDED THE ACFT AND DEPARTED. ON SECOND TKOF; THE SAME VIBRATION RETURNED WITH THE SAME RESULTS. ON RETURN TO THE ARPT; I NOTICED ANOTHER MAP SHIFT THAT WAS SOMEWHAT LARGER THAN LAST TKOF. AND ON SHORT FINAL; THE GND PROX STATED 'TOO LOW; TERRAIN; TOO LOW; TERRAIN.' I TRIPLECHKED THE GEAR AND FLAPS AND ALL WAS AS REQUIRED FOR LNDG. REST OF LNDG WAS UNEVENTFUL. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED AN ATTEMPT WAS MADE TO FIND THE CAUSE OF THE VIBRATION BY USING THE MAINT COMPUTER BUT THE CORRECTION HAD ONLY A 1-LINE ENTRY AND APPEARS THE START OF A NARRATIVE THAT WAS NEVER FINISHED. MAINT DOES NOT RPT ANY FINDINGS OR MAINT ACTION TAKEN TO THE RPTING CREWS.