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I flew last week on a Boeing 737-800, I was in row 7, so just in-front of the wings.

I know from experience that there is a significant difference in engine noise from the front to the rear of the aircraft, up front you get more of the buzz, towards the back you get more "roar" engine exhaust noise.

However on this occasion the thing that struck me was an abrupt change in noise at about 1 minute from takeoff, not sure on altitude, but probably around 2000ft.

The takeoff (EMA) was 100% normal, straight climb no significant turn.

At this (estimated) 1 minute mark, the right-hand engine suddenly got louder, a harsher buzz, but seemingly no change in RPM (no pitch change). I would say inside the aircraft it almost doubled in volume. There was no obvious added vibration to the airframe.

The flight went on as normal (4 hours to ACE Lanzarote) and the noise subsided as we reached cruise.

What could cause this? A few things went through my mind: undercarriage retraction(?). Bird strike, blade damage, flame-out, resonance, internal changes in the engine such as guide vane angles, engine working loose!

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    $\begingroup$ I would suggest certain things were done my the pilot to cause such noise. :) $\endgroup$
    – Super
    Jul 9, 2018 at 22:55
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    $\begingroup$ How do you know the RPM didn't change? You could also have been flying noise-abatement procedures that required them to maintain lower noise levels around populated areas. $\endgroup$
    – Ron Beyer
    Jul 9, 2018 at 23:37
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    $\begingroup$ @RonBeyer - I can't know for sure, but the change was abrupt, and there was no change in the pitch of the noise (so unlikely it was a throttle change) $\endgroup$ Jul 10, 2018 at 8:50
  • $\begingroup$ It may have been a change in wing configuration, which could have affected the acoustics, perhaps with a bit more throttle added in. $\endgroup$ Jan 6 at 1:59

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The buzzsaw sound is usually the fan blades breaking the sound barrier.

Could be the elevation change, temperature, and humidity caused the sound to be more pronounced at that time, or perhaps the pilots increased the thrust on that engine due to cross winds or for some other reason.

Here's a youtube video with a similar noise:

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I experienced the same thing. As I was deplaning, I asked the F/O who was standing in the cockpit doorway. He stated that the airline had an agreement with the engine manufacturer that required a reduced-thrust takeoff (as long as runway length supported it) with an increase in thrust at 1500'. I was surprised because I thought all takeoffs were done at max/near max thrust. He said the procedure reduced engine wear.

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  • $\begingroup$ Unless there is a specific reason to do a max thrust takeoff (long range flt t/o at heavy gross wt, wet/short rwy, climb gradient requirements, anti-skid inop, etc.), it is very common for takeoffs (depending on the types and locations of operations) to be done with a reduction in max available takeoff thrust (Reduced Thrust takeoff/Flex takeoff/Assumed Temperature T/O). $\endgroup$
    – 757toga
    Jan 6 at 17:25
  • $\begingroup$ Good to know, thanks $\endgroup$
    – RetiredATC
    Jan 7 at 0:11
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The noticeable uptick in noise from the engines after a lull in noise could have been the pilots adding thrust to resume climbing the aircraft after a momentary level off to comply with an ATC instruction to maintain a particular altitude followed by a clearance to resume climb to a higher altitude.

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This is the sound of take-off trust being reduced to the climb trust. Usually reduced at 1500ft. It can also work in the opposite way where there would be an increase in trust and engine noise.

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  • $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$
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    Jan 5 at 16:41

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