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.
Last edited by hikoushi; 2nd September 2024 at 07:28.