It's difficult to know without hearing the noise, but some possibilities:
Compressor blade interaction tones - the fan will be subsonic probably at approach power and internal turbomachinery noise more easily propagates against a slower axial flow. It will be fairly directional, so may only last a few seconds, but will sound like discrete tones at mid to highish frequencies (e.g. 80 blades at 4000 rpm shaft speed gives ~5KHz tone)
Combustion instability - a low frequency 'growl' or 'hoot' at around 500hz, more commonly heard at very low powers when starting up or taxying.
Airframe generated noise - this is almost the most dominant noise source on some big iron at approach powers. Flaps, slats, gear bogeys etc all generate vortices. Usually quite broadband and lowish frequency, but smaller items can generate more high pitched disturbances.
Handling bleeds - often open during power changes to offload the compressor by leaking air into the bypass duct. This can create a bit of a whistle on some powerplants.
I can't give you a definite, but maybe some food for thought.