Narrative:

Flight was progressing normally until; as we leveled off at FL340 and accelerated; we heard and felt the aircraft start to vibrate and rumble. In the cockpit; this rumbling came on rather fast. The captain and I were discussing something interesting and both of us froze and fell silent as the rumble started. I first thought it was mach buffet; but I've never experienced mach buffet in the airbus. I looked at the mach speed and it was only around .75. The captain then commented that he thought it sounded like mach buffet and he selected a lower mach of around .71.as the throttles came back; the aft flight attendant called and reported that she too was experiencing a very unusual buffet in the back of the aircraft. We pulled up the flight control page on the ECAM and observed no unusual indications. As we slowed; the buffet dissipated significantly. We informed ATC that we wanted to stop our climb. We had the flight attendant set up the cart; and I went back to inspect the rear of the cabin. The captain and I both agreed that it was in the best interest of safety to try to find out if the vibration was from the rudder or the elevator. Before I left the cockpit; the captain asked how I felt the flight should proceed. He finished the sentence with; 'I'm leaning toward going back.' I replied that I agreed; but I was really thinking oh thank god. I'm not going to have to convince this guy to do the right thing.as I reached the aft cabin I did observe a vibration in the floor of the aft galley. It felt normal to me; and I asked the flight attendant if this was normal vibration on most flights. She replied that it was not. I then called the cockpit; and the captain slowly increased the mach speed. As the speed increased; there was a point where the vibration increased significantly. The captain maintained that speed for approximately 20 seconds as I physically felt around the galley walls and ceiling to try and locate the source of the vibration. With all the plastic absorbing the vibration; I was unable to definitively determine the source. The galley floor was definitely affected; but that did not help my investigation. On the interphone I informed the captain of my lack of result; and he slowed the aircraft as I returned to the cockpit.as I reached the cockpit; the captain had already informed ATC of our return to departure airport and we were in a left 180. Before the turn was complete we had discussed how to proceed back to the airport with regard to speed and flap configuration and emergency status. The captain commanded me to declare the emergency and I did so; giving ATC sob's (souls on board) and fuel in hours. We agreed that the landing should be a normal flaps full landing but that it should be very stabilized and we should require the longest runway. ATC was informed of this too; while we simultaneously informed dispatch of our return and emergency state via ACARS. During decent; we discussed cabin advisory vs. Cabin prep; and decided that a cabin advisory would be sufficient for the situation. The call was made to the purser; and she was informed of this and the time remaining.emergency crews were requested upon arrival with both ATC via radio; and dispatch via ACARS. Overweight landing was discussed; but the gross weight (140.0) was already below the limit. I checked the landing distance on the card and determined that runway was more than sufficient for the landing. The quick reference diversion guide was checked with nothing missed or new. The flight proceeded very normally from FL180 on to touch down with the exception that we were set up very early and were slower. On 5 mile final; we informed tower of our planned runway exit so that the emergency crew could be aware of our plans. From 1;000 AGL the flight was a normal; everyday flight. As we cleared the runway; we canceled the emergency and proceeded to our gate with no further complications.

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Original NASA ASRS Text

Title: A rapid onset of airframe vibration at cruise Mach caused an A320 flight crew to return to their departure airport for maintenance.

Narrative: Flight was progressing normally until; as we leveled off at FL340 and accelerated; we heard and felt the aircraft start to vibrate and rumble. In the cockpit; this rumbling came on rather fast. The Captain and I were discussing something interesting and both of us froze and fell silent as the rumble started. I first thought it was mach buffet; but I've never experienced mach buffet in the Airbus. I looked at the mach speed and it was only around .75. The Captain then commented that he thought it sounded like mach buffet and he selected a lower mach of around .71.As the throttles came back; the aft Flight Attendant called and reported that she too was experiencing a very unusual buffet in the back of the aircraft. We pulled up the flight control page on the ECAM and observed no unusual indications. As we slowed; the buffet dissipated significantly. We informed ATC that we wanted to stop our climb. We had the Flight Attendant set up the cart; and I went back to inspect the rear of the cabin. The Captain and I both agreed that it was in the best interest of safety to try to find out if the vibration was from the rudder or the elevator. Before I left the cockpit; the Captain asked how I felt the flight should proceed. He finished the sentence with; 'I'm leaning toward going back.' I replied that I agreed; but I was really thinking oh thank God. I'm not going to have to convince this guy to do the right thing.As I reached the aft cabin I did observe a vibration in the floor of the aft galley. It felt normal to me; and I asked the Flight Attendant if this was normal vibration on most flights. She replied that it was not. I then called the cockpit; and the Captain slowly increased the mach speed. As the speed increased; there was a point where the vibration increased significantly. The Captain maintained that speed for approximately 20 seconds as I physically felt around the galley walls and ceiling to try and locate the source of the vibration. With all the plastic absorbing the vibration; I was unable to definitively determine the source. The galley floor was definitely affected; but that did not help my investigation. On the interphone I informed the Captain of my lack of result; and he slowed the aircraft as I returned to the cockpit.As I reached the cockpit; the Captain had already informed ATC of our return to departure airport and we were in a left 180. Before the turn was complete we had discussed how to proceed back to the airport with regard to speed and flap configuration and emergency status. The Captain commanded me to declare the emergency and I did so; giving ATC SOB's (souls on board) and fuel in hours. We agreed that the landing should be a normal flaps full landing but that it should be VERY stabilized and we should require the longest runway. ATC was informed of this too; while we simultaneously informed Dispatch of our return and emergency state via ACARS. During decent; we discussed Cabin Advisory vs. Cabin Prep; and decided that a Cabin Advisory would be sufficient for the situation. The call was made to the Purser; and she was informed of this and the time remaining.Emergency crews were requested upon arrival with both ATC via radio; and Dispatch via ACARS. Overweight landing was discussed; but the gross weight (140.0) was already below the limit. I checked the landing distance on the card and determined that runway was more than sufficient for the landing. The quick reference diversion guide was checked with nothing missed or new. The flight proceeded very normally from FL180 on to touch down with the exception that we were set up very early and were slower. On 5 mile final; we informed Tower of our planned runway exit so that the emergency crew could be aware of our plans. From 1;000 AGL the flight was a normal; everyday flight. As we cleared the runway; we canceled the emergency and proceeded to our gate with no further complications.

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.