Transmitter RF amplifiers can generate parasitic oscillations:

Because parasitic oscillations rely on random stray capacitance and inductance in a circuit's physical layout, their resonant frequencies are entirely unpredictable. Unlike harmonics, which are strictly mathematical multiples of the operating frequency (and therefore always above the fundamental frequency), parasitic oscillations have no fixed mathematical relationship to the desired signal. As a result, they can appear anywhere—either above or below the transmitter's intended operating frequency. They are not restricted solely to VHF frequencies, even though VHF parasitics are common. Furthermore, they do not occur on the fundamental frequency, because that frequency is your actual desired, controlled signal.