i think;-
1) u didnt do ur grounding properly, loose grounding also will create aeroplane sound.
2) or ur rca cables running parallel together with power cables
3) battery weak n faulty alternator also can lead to aeroplane sound

No worries, there r lotsa culprits which can lead to aeroplane sound, even experience installer also facing this kinda problems. If installation no problem then most probably either amp, hu or crossover.

Bro, altho ur alternator still charging n working, its doesnt mean that it wont create aeroplane sound, some install helix alternator cap to overcome it.