From my experience, admittedly pretty subjective, is that a spoke is about to fail. Nearly every time I've noticed that a wheel has developed a bit of wobble, within a couple hundred miles I'll get that distinctive "ping". In the old days wheels going out of true was common and you could correct it easily. But today's wheels seem much stronger and normally stay true for thousands of miles, but then a spoke will stretch just a bit and then fail.
I'm not an expert, just my experience.
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