Local high-power FM transmitters, mobile phones close to stereos and other radio transmitting devices will emit high-frequency radio waves to the surroundings when they are used, and the transmission process of radio waves will generate high-frequency induced current on metal objects. The audio input line will also generate induced current, which will be amplified and frequency-discriminated and coupled to the power amplifier, so the speaker will have interference sound.
Many audio input wires are not shielded wires, but are replaced by ordinary two-core wires, which opens the door for high-frequency current.
Solution: Find a core wire with approximately the same length as the input wire, with a barbed wire outside and a shielded wire with insulation outside. At the input point inside the stereo, connect the wire mesh to the original ground, connect the core wire to the input, and connect the plug at the other end of the shielded wire, and the problem will be solved.