“What was that?” It’s a common phrase uttered from passengers’ lips….and with good reason. In the controlled, managed world of aviation, we don’t anticipate surprises, emergencies, or even unfamiliar noises. So what WAS that noise?You needn’t have a fear of flying to be concerned. The truth is that from first taking your seat to the point at which you step from an aircraft and onto an aerobridge, all manner of sounds can emanate from an aeroplane going about its business. As we wait at the terminal, a flurry of activity is taking place beneath the floor as tonnes of baggage, freight and even livestock are loaded for the flight ahead. The movement of these pallets and the positioning of individual bags can lead to sounds rising up through the floor. Not to mention the barking and whimpering of reluctant pets being put in their place.On the rare occasion, the contents of these storage spaces, or ‘holds’ can shift slightly. It may just be an isolated package or bag and not enough to have any impact upon the weight and balance of the aeroplane. Still when any noise comes from below, it can be disconcerting. More regular is the sound of the landing gear and the flaps on the wings extending or retracting. I say regular as these are part of the normal process for any airliner taking off or landing and are generally associated with the early and latter stages of flight. These sounds can still vary based upon the speed at which they are deployed and where the mechanisms that drive them are located. Often located mid-cabin beneath the floor, the driving motors and moving parts can make various noises that attract the attention.In between times and particularly when descending to land, the pilot can extend panels from the upper surface of the wing. These can be readily seen and serve to spoil the smooth airflow over the wing and are consequently known as ‘spoilers’. The act of disrupting the airflow with spoilers can also be associated with a low rumbling and a degree of vibration. And depending on how smoothly, or not, the spoilers are extended and retracted there can be a rising or sinking feeling felt in the cabin. Air conditioning systems can be another source of noise in the cabin. In order to keep a comfortable, oxygen-rich atmosphere in the cabin of most airliners, a ‘pressurisation system’ is utilised. This allows the passengers and crew to fly in the thin air of the upper atmosphere without the need to wear an oxygen mask. It is also why when the system fails that masks drop from the roof to provide supplemental oxygen while the pilots descend the aircraft to a safe altitude for normal breathing.As with any system that moves air through pipes, it can create noise and occasionally there can be sounds associated with subtle surges of airflow. These will often be felt as a pressure change in the ears as well. Different aircraft will have different characteristics of this air noise and on occasions, some types will reroute the air-conditioning system increase take-off performance. This can result in the sound of rushing air through the overhead ventilation ports, particularly as the pilots reconfigure the system after take-off.Air-conditioning, spoilers, flaps, landing gear and cargo. Even the aerobridge can groan as it automatically resets as you disembark. These are all noises that can peak a passenger’s interest but are actually par for the course in aviation. Have you heard a noise you can’t explain? If so, contact me through this website and I’ll see if I have an answer for you.