LOPT squawking at HALF line frequency

[Terry]

If whatever is producing the sound is resonant at a frequency where the length of one cycle is longer than one line length but shorter than two, say, but the Q is low enough not to keep it ringing at the end of one cycle, then if one flyback pulse kicks it off, it is still part way through its cycle when the next appears, so is ignored. It then completes its cycle and waits for the next flyback pulse, and so on.

You say it squawks rather than sings, so could that be a pointer to its discontinuous sequence of individual pulses?