Motor noise
Motor noise is vibration from the motors and props (imbalance, bearing wear, frame resonance) that shows up in the gyro signal as peaks that move with motor RPM.
In the FFT it appears as sharp peaks that shift with throttle. If filters don't remove it, it heats the motors and corrupts tracking. PIDoctor overlays motor-harmonic lines on the spectrum and checks whether your RPM filter / dynamic notch actually attenuate them.