What is with that noise on these planes. When the plane has come to a complete stop at the jetway, there’s some kind of rhythmical sound as they shut something off. For the life of me it always sounds like someone on the ground is taking a very big hacksaw to a major portion of the plane.
I’m sure it has something to do with hydraulics, but what specifically are they doing to make this noise?..other than hacksawing off a used tire or wing or something.
Never heard a hacksaw described as “rythmical”. Who knows, it could be the electric motors that open the cargo doors on some larger aircraft.
Most likely retracting the flaps.
It’s the treadmill spinning up in order to keep the plane stationary.
Seriously though, I’ll go with retracting the flaps.
The flaps should be retracted while the plane is still rolling on the runway. I’ll go with opening the cargo doors.
Could it simply be the engines spooling down?
I don’t fly a lot, about once a year. But I know just what you are talking about. Sounds like a Sawzall taking on lose metal with a wood blade.
I wonder too.
More accuratley, I’m guessing it’s the hydraulic pump working to retract the flaps.
I’ve never been on a flight where the flaps have not been retracted long before the aircraft reaches the gate.
Um… yeah. 
Did I say something otherwise? As the plane is clearing the active runway, the pilot will run through his afterlanding checklist, and one of the steps will be to raise the flaps. He’ll move the flap handle. That will actuate the hydraulic pump, which will raise the flaps.
The OP’s noise happens after the aircraft stops at the jetway, so it’s probably not flaps.
Verifying (flew A320 series several times last 6 months): the noise being referred to happens after jetway stop, all surfaces retracted, engines spooled down, aircraft running on APU. Sounds indeed like an unskilled worker using a saw, kind of a broken rhythm. Or else like a very large dog, trapped inside a metal can, barking.
Drives everyone batty.
RRRRRETTTTT-rrrrewwww
RRRRRETTTTT-rrrrewwww
RRRRRETTTTT-rrrrewwww
RRRRRETTTTT-rrrrewwww
RRRRRETTTTT-rrrrewwww
RRRETT-rreww
RRRETT-rreww
RRRETT-rreww
RRRETT-rreww
RRRETT-rreww
RRETT-reww
RRRETT
RRRETT
It’s the gremlin from the Twilight Zone episode…
It is most likely the Power Transfer Unit (PTU) which transfers hydraulic pressure from one system to another when the second system loses pressure. It seems that it can be heard both just after engine start and after parking.
http://crankyflier.com/2007/02/22/what-is-that-noise/
That second website is a good place to ask these kind of questins, though the signal to noise ratio isn’t up to the same standard as the SDMB.
Good point. Missed that. Then I have no clue what the noise is. :o
I’ve heard that on 319/320’s too… The noise is always while parked at the jetway.
I thought it might be the baggage bins coming up or down.
I’m pretty sure 1920s Death Ray has it.
The engines supply power to the hydraulic systems. Sometimes an aircraft will taxi on one engine to save fuel, in which case Airbus aircraft use a device called a power transfer unit to cross-pressurise one hydraulic system from the other engine. That’s what makes the alarming noise.
The same issue exists on Boeing aircraft, but they use a different technology.