Notch - Filter
With some sets, the notch filter acts on the radio frequency level; with others, a whistling signal is eliminated on the audio signal level.
After switching on, the knob of a variable notch filter must be turned sensitively until the whiste disappears. The AOR 7030 has an automatic notch function; the receiver automatically detects a disturbing whistling tone and fades it out.
Some other (simpler) radios have an audio filter to eliminate a 5 kHz whistle generated from a station on an adjacent 5 kHz channel.