Tech log entry for this type of event could be applicable. Typical result might be "ground checked, no fault found, Further report please". The writer of the original report will need to respond again if the problem remains. Other crews will see the continuing fault(?) but no need to write it up as the originator seems to be persistent. After a few months originator starts to think its his imagination as no-one else is reporting.
Unreported matters and those that are difficult to diagnose, can still be significant, and there is an example of loss of aircraft from non reporting. In a case that I know of, a persistent lone voice prevented an almost certain loss.

Same sound heard through whole flight in cabin
 

Dear all

I flew with a 5yr old A320neo today. The noise started halfway through the taxiing and stopped only after the ignition was switched off.

I recorded the noise where it can be heard clearly but I dont know how to add it here.

Does anyone know what causes this noise?

Thanks

Audio added
 

sndup. net / mct38

(Please delete the spaces as I dont have 8 posts yet and cannot add links yet)

What you hear is the PTU. Since it started mid-flight and continued throughout most probable some lowish pressure in either the green or yellow hydraulic system developed

So the sound which you described is something else then, right?

The noise (PTU according to your expertise) started on the ground while taxiing to takeoff.

Why did the pilots take the risk and still took off without fixing the issue first on the ground?

Was that something illegal? Did they put the people in risk?

The PTU went on long before the flight started and stood on until after the landing.

The sounds this thread is referring to is a different one, correct.

You'll find a few topics regarding your PTU behavior, better have a look at those if you wish.
Nobody is risking anything, but in order to not be grounded for every small misbehaving system we have something called a MEL where it's clearly outlined what system might be faulty and under which conditions.

Since the job of the PTU is basically to help pressurise either the green or yellow hydraulic system the reason for a constantly running PTU could be as simple as a faulty pressure sensor, so that the PTU "thinks" it's time to get to work. There is a MEL for that which makes you check that the PTU is running properly. It doesn't mention it has to actually stop running at some point, which (if you're trying to understand the logic behind those MELs) it might be doing just for passenger comfort and mechanical long term benefits anyway. But if it does its job whilst running continuously - there's no harm to go fly with it.

Back to 'your' problem; maybe it's the Blue Hyd pump ? Maybe its mountings are slightly loose and causing various resonances - or a hose connected to it is vibrating against something. Not sure quite why this would change with altitude, although of course temperature does change with Alt. and the airframe cools down during the cruise.

Happy flight attendant? :8

The GTF does make some interesting noises - I have noticed it when flying on NEO's equipped with them and when watching them taxi past. There is a noticeable hum/resonance not far above idle thrust, I've heard that it's related to resonance in the combustor or similar, happy to be corrected though (it's not an engine I've worked on).

There is a very noticeable harmonic resonance type sound during power changes from idle to greater than idle, both on the ground and airborne. Those are primarily caused by the temporary out-of-sync condition of the geared fan as its speed stabilizes relative to the rest of the engine. The GTF is a glorified Pratt and Whitney turboprop engine, and this lovely noise is kind of like when you are flying behind a PT6 or PW121 and your ham-fisted partner jams the condition levers forward on approach and the governors and synchro phase takes a second to catch up. The romantics like to say it sounds like a whale call on the ground. But if you stand under one on final approach and the ham-fisted NEO pilot (or A/THR) goes from idle straight to 50% N1 (approach power), it sounds more like the plane is breaking wind. Loudly.

if you really want to make the airplane “crack one” to someone you know who lives between 2.5 and 4 miles out from the airport right under or slightly next to the final approach of your favorite runway, time your decelerated approach to be flight idle and reach a few knots above Vapp EXACTLY abeam that person’s house. Then advance the thrust levers directly to the position corresponding to 50%N1. Do it promptly, with no finesse. It should spool the fan when you are about 10-20 degrees past their position, and it will sound like God’s own flatulence. This is precisely opposite of a noise abatement procedure, and it WORKS.

Ask me how I know!

But seriously, assuming the APU is not running (a APU bearing about to s**T the bed would be my first thought) the OP’s issue sounds like others have said, a cabin fan or fuel / hyd pump about to go. Have also heard some very strange and persistent sounds from the air conditioning if there is something partly blocking a duct. Call it the “booger whistle”. Really have to compare to another tail and see if it is unique to that aircraft.

Thank you so much for taking your time and explaining my concern in a very detailed way.

It seems that Im not the only one who gets worried when hearing this continous noise in the plane xD

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