Interference  --  Broken shielding, missing ground wire, corrosion, and fraying at the connector cable to the microphone can cause electrostatic induction yielding a stray magnetic field or voltage into the recording chain. The interference can be at an audio frequency or high frequency. All inputs must be shielded. Shielded transformers attenuate passage of interference. Symmetrical inputs in which the middle point is grounded and external filters protect against induction leaks. Magnetic induction is caused by use of microphones near motors, transformers, twisted power cables, or other elements like auto ignitions, refrigerators, lighting grids. This source of noise tends to record as low frequency hum.