In Morse code transmission, local RF interference (key clicks) is produced by:

When discussing Morse code (CW) transmissions, it is important to distinguish between local interference and distant interference. Local RF interference, often heard on nearby receivers or stereos in the exact same room or house, is typically caused by the physical making and breaking of the electrical circuit directly at the Morse key contacts. As the physical contacts open and close, tiny arcs or sparks can form. These sparks generate broad-spectrum, low-power radio frequency bursts that easily interfere with nearby electronics, functioning much like a miniature spark-gap transmitter. This phenomenon is completely separate from distant signal issues like poor wave-shaping or voltage regulation, which affect the actual transmitted RF carrier heard miles away.