I often hear a “barking” or “woofing” sound on airplanes after I land. I’ve always assumed that it’s the sound that some ballscrews make, but I don’t know what they are adjusting. I’ve never been able to see any flaps or wing extensions moving.
Anyone know what makes this sound (it seems to come from under the floor)?
It’s entirely possible that there are dogs in the hold:
It is probably the Power Transfer Unit that you’re hearing. Basically, it maintains hydraulic pressure especially when only one engine is running.
As far as I can tell, it’s just the twin-engined Airbus models that make the noise.
Yes, that’s it, thank you.
I just got off an A320, and was reminded about asking the question, after hearing it.
Yes. There are actually many aircraft that are capable of making that, or a similar, noise, but it seems to be the Airbusses whose design means it is regularly heard by passengers.
+1
This year, we heard precisely this sound on an Airbus A320 flying to Lisbon and I’d never heard it before. As always, I glanced at the crew and they were very matter-of-fact so I ignored it.
It was very loud and I wonder if it is far more noticeable where we seated? (just aft of the wing)
And I think the plural of airbus is airbussi 
Example of what the OP is hearing:
Absolutely the PTU.
The joke with Airbuses is that the noise is the dog that helps fly the plane. An Airbus needs three things to fly: a computer, a pilot, and the dog. The computer flies the aircraft, and the dog bites the pilot if he touches the computer.
I had a question a few years ago on this exact issue
The good old "scare"bus 320
I keep hearing this, I dunno, epithet? Why on Earth would you be scared on an A320?
A curiosity on Scare Bus.
Friend works for AA and when they open one up on “C” checks and stuff you can tell who built which parts of the airplane.
Rivets change style & brand, castings are different, the German parts are built way better than other parts, etc…
He points out that usually when an metal plane starts to go bad you have rivets that are lose, flanges and braces start bending, all kinds of things even the worst mechanic can see and ask about.
The composite ones do not do that, they just fail catastrophically.
XRAY you say… Most of the critical parts are buried deep and some have to be removed to be inspected. $$$$$$$$$$$$ and time not producing revenue.
There a few things that if they have not been corrected, IMO, make it very hard for pilots to respond safely to a few different situations.
The big iron drivers will be along to tell me I am full of it. That it is a safe airplane. Define safe. ( for how long )
Look to Lockheed for design.
Loot to Douglas for engineering.
And look to Boeing for dam fine airplanes.
If it ain’t Boeing, I ain’t going.
This is a bit dated but so am I. 
Time will tell. I am just waiting. Hope it happens on the ground so no one will die.
And maybe they will catch every dangerous thing before it hurts anyone.
The first A320 was delivered over a quarter of a century ago. How long are you going to wait?
And yet it’s Boeing that has produced the worlds first predominantly composite aircraft, the B787.
See, I knew the big iron drivers would make fun of me.
Question for A B drivers. When flying small airplanes, if you ever do, do your feet stay on the floor during an unexpected control emergency’s and the plane is going into an unusual & dangerous attitude?? What does the book say? “First wait for the autopilot to fix it?” * cheap shot *
Do the new A B’s have a lock out for the rudder now. No more vertical stabilizers going to break?
Well, judging by my age which you Shirley know, and that the 787 was not around when Douglas was, that I would not like the 787 either.
“Lets be careful out there.”
And the pilot feeds the dog. (Or else he needn’t be there at all!)
The dog is always busy, too. It’s constantly running, … on a treadmill.
I hear it can be soothed by playing Duran Duran’s “Rio.”