Narrative:

We were climbing through FL210 toward our cruise altitude of FL320. The flight plan planned a 335 kts/.80 mach climb; to cruise .80M. The captain was flying with the autopilot engaged; and we were accelerating through 320 kts toward our planned climb speed. The air was smooth. Through FL210; we began to experience a little light chop; so; I selected 300 kts; and dialed it in. The turbulence increased to continuous light chop; then we began to feel a rhythmic; continuous vibration. The vibration was a vertical up and down motion; and was steady. It was something I had never felt before in my years of flying. It definitely was not a meteorological condition. It felt like driving a car on a washed out dirt road with washboard potholes. I dialed back the speed toward 275 kts; and as the airplane slowed through 300kts; the vibration ceased. The light turbulence was still apparent; but; the vibration was gone. The aft flight attendants called the cockpit wanting an explanation for the event; and the first officer took the call. The forward flight attendant said she didn't feel a thing; but; the aft flight attendants said they had never felt or heard anything like that in 30 years of flying. We continued our climb and accelerated again; very carefully; but the vibration was gone. We sent a message to [maintenance] regarding the event. We attempted to duplicate the issue on descent into ZZZ; but the ride conditions were smooth; and the aircraft responded normally; no vibration was felt.

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Original NASA ASRS Text

Title: A319 Captain reported an unusual vibration was noted during climb; but it disappeared after speed was reduced.

Narrative: We were climbing through FL210 toward our cruise altitude of FL320. The flight plan planned a 335 kts/.80 Mach climb; to cruise .80M. The Captain was flying with the autopilot engaged; and we were accelerating through 320 kts toward our planned climb speed. The air was smooth. Through FL210; we began to experience a little light chop; so; I selected 300 kts; and dialed it in. The turbulence increased to continuous light chop; then we began to feel a rhythmic; continuous vibration. The vibration was a vertical up and down motion; and was steady. It was something I had never felt before in my years of flying. It definitely was NOT a meteorological condition. It felt like driving a car on a washed out dirt road with washboard potholes. I dialed back the speed toward 275 kts; and as the airplane slowed through 300kts; the vibration ceased. The light turbulence was still apparent; but; the vibration was gone. The aft flight attendants called the cockpit wanting an explanation for the event; and the FO took the call. The forward flight attendant said she didn't feel a thing; but; the aft flight attendants said they had never felt or heard anything like that in 30 years of flying. We continued our climb and accelerated again; very carefully; but the vibration was gone. We sent a message to [maintenance] regarding the event. We attempted to duplicate the issue on descent into ZZZ; but the ride conditions were smooth; and the aircraft responded normally; no vibration was felt.

Data retrieved from NASA's ASRS site 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.