In this video of a C42 landing, there is an audible, intermittent buzzing noise that persists until landing, at the 59 second mark, it reaches its loudest peak.
What is the noise? It's not a stall horn, as the C42 doesn't have one. It sounds electrical, but I'm not sure. I have noticed this on most C42s I have flown, so this is not particular to this video or aircraft.
It's nothing mechanical other than possibly a loose fairing. Not falling off loose, but loose enough to vibrate in the air stream.
It sounds a little bit like the reed type stall horn used on Cessnas. They have a hole in the leading edge, and above a certain angle of attack, the air blast entering the hole activates the reed in the duct leading into the cockpit, with a similar, but much louder sound, a bit like the little horn on a scooter. So it's not from that.
The most likely source is a piece of plastic or sheet metal, with a gap between an edge and a surface, creating the same sort of effect as air blows into or around it when AOA or yaw angle is just so. Probably, somewhere in the fairing materials around the windshield, wing roots or doors, a small hole or gap is causing some piece of plastic or aluminum to resonate at that frequency and make the sound.
Check for loose fairings/fairing strips. If nothing is amiss, not much to do unless it really bothered you and you owned the plane. In that case, if I was trying to eliminate it, I'd be looking for gaps in fairings etc, exposed flexible edges, and taping them over and trying out the result.
It's very hard to tell the source of a sound from a video alone, it's hard enough at the time when you can at least perceive where about a sound is coming from.
I could make some wild guesses, as with anything like this if it concerns you get a qualified engineer to check it over.
Metal airframes have some "play" and you often, especially in gusty conditions, get some vibration in the plates. A slightly lose rivet will allow some vibration between parts sometimes heard as a whistling or buzzing sound
Engines are held in place by multiple bolts, if one is slightly lose or missing, the engine may vibrate because of the spinning of the prop. get all the engine mounts checked or eyeball them yourself. It's a useful skill to be able to identify all the engine mounts - I'd want to check them before every flight if possible.... not always easy with some cowlings.
Baggage. Could there have been two containers in any baggage area touching and causing a buzzing sound due to vibration? This is probably the most likely candidate if I'm honest!
Lastly, check your prop, even a decent size notch gouged out can unbalance it and cause extra vibration leading to one or more of the above.
