From the Feb. 19, 2019 ARRL Contest E-Letter

WORD TO THE WISE

Key Clicks

Key Clicks are caused by the undesirable and unwanted widening of a
CW signal, due to energy in the signal's sidebands. Contributing
factors include the shape of the keying waveform, too-short rise and
fall times of the transmitted signal, or variation in the carrier
oscillator frequency.

Key clicks can sound like thumps or clicks to a receiver tuned with a
few kHz of the transmitter frequency. Unscrupulous operators have used
key clicks to their advantage by making adjacent frequencies undesirable
to use, seeking to gain an unfair advantage in crowded contest
conditions. This is against the rules of most contests.

As a side note, it can also get you a notice of violation from either
an OO (Official Observer, soon to be Virtual Monitor (VM)), or from the
FCC.
--- SBBSecho 3.20-Win32