Narrative:

Takeoff was uneventful as first officer was pilot flying. Passing 10;000 feet; de-sterilized flight attendant's (flight attendants); turned off lights as we continued climb. Switched from [departure] freq to center. Upon check in with center; appx 12-15000 ft; I noticed a medium burning electrical smell; didn't mention anything at first to see if smell would dissipate; it didn't. Appx 3-5 mins later; passing FL180 in climb felt a continuous strong vibration under my floorboard followed by a loud 'thud' which sounded like a metal door hitting a door frame. It was at this point that the burning smell got worse. I told the first officer (first officer); he has the [aircraft] while I called the #1 flight attendant. I asked him if he heard or smelled anything. He did in fact confirm the strong vibration coming from under first class galley. He did not smell anything but the #4 flight attendant in back of plane said she smelled a light burning odor. It was at this point that I realized something was going on. I told first officer we're going to turn back to [the departure airport]. I told [center]; we need to head back for a mechanical problem. I then called #1 flight attendant back and told him we are returning; no evacuation; planned and will be on the ground in 15 mins; he had no questions and I told him to start getting cabin ready for landing.upon landing; and talking with our local company maintenance; he concluded quickly that one of our blower fans for extraction valve for avionics cooling had malfunctioned.

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Original NASA ASRS Text

Title: A321 Captain reported returning to departure airport after experiencing vibrations and a burning smell.

Narrative: Takeoff was uneventful as First Officer was pilot flying. Passing 10;000 feet; de-sterilized FA's (Flight Attendants); turned off lights as we continued climb. Switched from [Departure] freq to Center. Upon check in with Center; appx 12-15000 ft; I noticed a medium burning electrical smell; didn't mention anything at first to see if smell would dissipate; it didn't. Appx 3-5 mins later; passing FL180 in climb felt a continuous strong vibration under my floorboard followed by a loud 'Thud' which sounded like a metal door hitting a door frame. It was at this point that the burning smell got worse. I told the FO (First Officer); he has the [aircraft] while I called the #1 FA. I asked him if he heard or smelled anything. He did in fact confirm the strong vibration coming from under first class galley. He did not smell anything but the #4 FA in back of plane said she smelled a light burning odor. It was at this point that I realized something was going on. I told FO we're going to turn back to [the departure airport]. I told [Center]; we need to head back for a mechanical problem. I then called #1 FA back and told him we are returning; no evacuation; planned and will be on the ground in 15 mins; he had no questions and I told him to start getting cabin ready for landing.Upon landing; and talking with our local company maintenance; he concluded quickly that one of our blower fans for extraction valve for avionics cooling had malfunctioned.

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.