EMB-135 CREW HAD AN ACFT VIBRATION BEGINNING ABOUT 300 KTS. THE CREW DID NOT LIST THE VIBRATION IN THE ACFT MAINT LOGBOOK.

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AFTER DEPARTING FROM LGA AND CLBING THROUGH 11000 FT MSL; I BEGAN LOWERING THE NOSE; DECREASING RATE OF CLB WHILE INCREASING AIRSPD FOR A CRUISE CLB. AS AIRSPD BEGAN TO INCREASE TO AROUND 300 KIAS; WE BEGAN TO FEEL A VIBRATION IN THE AIRFRAME. I WAS STILL HAND FLYING AND THE VIBRATION DID NOT SEEM TO BE COMING FROM OR EFFECTING ANY CTL SURFACES. THE ACFT HAD A CDL ITEM FOR A MISSING APU ACCESS DOOR WHICH IS A FAIRLY LARGE DOOR ON THE TAIL OF THE ACFT. AFTER INCREASING CLB RATE; WHICH SLOWED THE ACFT TO BELOW 280 KIAS; THE VIBRATION STOPPED. AFTER DISCUSSION; WE DECIDED THAT THE VIBRATION WAS MOST LIKELY DUE TO THE MISSING APU ACCESS DOOR AND THAT WE WOULD CONTINUE TO OUR DEST. FOR THE REST OF THE FLT; WE DID NOT EXPERIENCE ANY FURTHER VIBRATION. WE FLEW TO OUR DEST AND BACK TO LGA WITHOUT FURTHER OCCURRENCE WHICH MAY HAVE BEEN DUE TO FLYING AT THE SLIGHTLY SLOWER AIRSPD. RECENTLY; THE CAPT WAS CONTACTED ABOUT A CURRENT FAA INVESTIGATION ABOUT AN OCCURRENCE WITH THE SAME ACFT HAVING A FLT CTL PROB A FEW WKS LATER. THEY WERE ASKING FOR ANY HISTORY OF PROBS WITH THE ACFT. THE CAPT GAVE AN EXPLANATION OF OUR OCCURRENCE. AS I HAVE JUST BEEN NOTIFIED OF THE POSSIBLE INVESTIGATION; I DECIDED IT WOULD BENEFIT THE NASA ASRS TO BE INFORMED OF THE OCCURRENCE.