Key clicks, heard from a Morse code transmitter at a distant receiver, are the result of:

When key clicks are heard from a Morse code transmitter on a distant receiver, they are caused by an issue with the transmitted RF signal's envelope, not physical sparks at the key. If the CW signal turns on (rise time) and off (decay time) too instantly, the waveform resembles a perfectly square wave. According to the principles of signal processing, a sharp square wave contains many high-frequency harmonics. In a radio transmission, this translates into wideband noise spreading out from the carrier frequency, sounding like clicking noises on adjacent frequencies. To prevent this, transmitters use a keying filter to slightly round off the edges of the CW envelope. This softens the rise and decay times to limit bandwidth without making the Morse code sound overly mushy to the receiving station.