Did you measure the distance between the poles? I suspect it’s different from all the other spans, so this one happens to have a resonant frequency that exactly matches whatever vibration source is already there (could be the tension too). As for sources of vibration, wind is probably it, even if it’s not strong. If it just happens to create the right frequency, the cable will vibrate just like a violin string.
I have 4 kids and I don’t want to pay over 100k for a car. Believe it or not, there is currently no EV option on the market. It’s looking like the EV9 will be the first…