Just a general question about engine noise on approach vs. high altitude cruising. I’ve noticed that on approach, jet engines make more of a rumbling and rougher sound than they do at cruising altitude. Is it just because they’re running at a different RPM level, similar to being downshifted, say, from 5th/overdrive gear to third? 
Thanks!
I’d say the difference in noise you notice has more to do with the inherent high-drag configuration during approach and low-drag configuration at cruise altitude, and less to do with the RPMs of the actual engine. (By the way, jet engines don’t employ any transmission in the way an automobile does).
That’s my feeble attempt 
I’m sure one of the experts on here will come fill you in.
Edit: This probably should have been posted in “General Discussion” and not “General Aviation”.
Thanks for the info! I was using the auto transmission comparison facetiously.
Are you talking about the sound from inside the aircraft or the sound from the ground?
A lot of the low, rumbling noises are caused by the flaps and slats. About 30 seconds after takeoff, watch the flaps and leading edge devices retract and notice the way the rumbling becomes much less once they are retracted.
I’m referring to the engine noise from inside the cabin. When at lower altitudes, say 15-20 minutes before landing (or thereabouts - maybe earlier), a transition in the noise seems to occur from the smooth, constant hum of high-level flight to a rather low, rumbling sound. The plane also feels different, rather “sluggish,” as if it’s being flown manually, although I assume that during a normal flight the autopilot is still engaged at that point. I’ve noticed this transition often, if I’m paying attention. I thought maybe it has to do with the change in atmosphere at lower altitudes.
My plane is a lot noisier, air over the windshield, at 3000 than at 12,000.
descending below 29,000ft we “transition” from a Mach number to Airspeed. from 29,000ft to 10,000ft (or 14,000ft in my case) we go as fast as that barber pole lets us, in the Lear the cabin noise is about twice as bad when you get above 300kts.
What you are hearing is the air flowing over the airframe. When you get real close to the ground you are going slower BUT there is increased drag do to flaps and landing gear and other drag devices, so they need more thrust and now you hear the engines
I always wondered about this.
On a MD-80 (maybe others) you hear air blowing though the system. But just before touchdown, in the flair, the air stops for a few seconds.
Don’t know it it happens in the back but in the front rows it always happens. What’s up with that?
Pull the engine to idle and the amount of available bleed air goes away.
Shields, What type of jet are you talking about? Some engines like the CFM on 737s make a droaning Whaaaaaa hum noise or almost grinding noise at reduced power. The sounds you’re describing sound more like drag from flaps and gear as has been stated.
The flight I took in an A319 over the weekend got me to thinking about this phenomenon, although I’ve also heard this same rumbling noise in Boeings. I wouldn’t describe the noise as grinding, per se, but more like muffled Harley Davidsons at idle.
Is anyone reading this post a commercial pilot? If so, is it true that, before landing, autopilot is engaged until the last 80 feet or so of altitude? When is autopilot engaged after takeoff?
I’m not a pilot, just a plane enthusiast.
We don’t allow their kind ‘round these parts! Gotta’ protect the women and livestock.
Wow, I started writing a reply like this, but it got too long and I just gave up! I’m so glad you guys are here to save me from having to form complete thoughts.
What airline was the A319? I think they have two different engine mfgs.
http://www.youtube.com/watch?v=tZtLKGculVc ffwd this video to 1:00… lots of grinding.
http://www.youtube.com/watch?v=FU1VOo_tQ6k This approach video starts out nice and quiet. As they apply flaps and landing gear you can hear the sounds increase. The engines wind up and down and the air speed changes affecting the air noise.
It was a flight on Spirit Air.
The mixed flow cowl on the IAE2500s on A32x sounds a bit different than the CFM56s on 737s and some A32x.
Depends on the pilot, the weather, and the company. On departure, I typically hand fly the airplane to somewhere around 10,000 ft before engaging the autopilot. If it's nasty weather, I might turn it on as low as 1000 ft to allow me to focus on the radar or just to give the PAX the smoothest ride.
On approach, if the weather is good I typically use autopilot until we are cleared for the approach or until a bit outside the traffic pattern, typically between 5000 ft and 2000 ft. If weather is poor, we usually let the AP fly until we have the airport in sight.
Not sure what the exact SOPs are for the airline guys these days.
I’m trying to match your description with my experience as a commercial passenger. First off, all the abrupt changes in sound must result from changes in what the airplane is doing, not the air. The changes in the air typically are gradual over minutes.
I’ll start with your 15-20 minute estimate. That’s about when the pilot throttles back, leaves cruising altitude, and begins the descent. (Actually I’d say it’s 20-25 min.) The engines seem to come back to almost idle; maybe an exaggeration. That makes it quieter in the cabin, but maybe you also notice rushing-air sounds over the engine sounds.
About 5-7 min before landing, the flaps and slats come out, often accompanied by whiny motor sounds (Airbus?). Flaps make even more rushing-air sounds because of the added drag. That changes the flying characteristics a bit, but mostly you’ll notice some slowing and maybe the nose going down a bit. The flaps might be extended in steps over several minutes.
About 1-2 min before landing, the landing gear comes out, with substantial thumping sounds, and then you have an actual roaring sound, caused by drag and turbulence from the gear. And the flying characteristics change again; it seems to wander more. Because it’s going slowly, crosswinds and up/downdrafts have more effect on the aircraft too.
I can’t think of many other noises during the descent, until touchdown.
Does that match your recollections?
Speed brakes/spoilers are often used during descent (at any altitude or distance from the airport) to control airspeed and vertical speed. They create a bunch of turbulent air that buffets the wings and airframe creating a very noticeable low frequency rumbling noise and vibration.
As do the slats and flaps… 
The Airbus A320, I believe is the Spirit Airlines type of aircraft that the OP is referring to. On the A319/20/21 the Flap/Slat drive motors, and main landing gear actuators/uplock mechanisms are very quiet compared to a Boeing product. The Boeing products have very "whiny"sounding drive motors and clunky main gear uplocks.
But the changes in aerodynamic acoustics can be directly attributed to the drag caused by the extension of slats, flaps, and landing gear during the landing phase of the flight.
During take off, the most noticeable sounds are created by the engines. When sitting next to or in front of the engines as full power is applied at take off the engine’s fan blades can create a very marked whining resonance. This is caused by the engine attempting to gulp more air than can be rammed into it at a slower aircraft speed. At cruise speed this virtually goes away as the engines are getting more air forced into them to swallow.
When sitting behind the engines at take off the engines create a strong rumble as they force huge amounts of air out. At cruise this rumble diminishes as the air forced out of the engine mixes more smoothly with the high speed air passing around the engine.