I had a root cause once that was a cable draped across the corner of a ventilation duct. The duct would vibrate when the air was on, and the corner was pretty

I had a root cause once that was a cable draped across the corner of a ventilation duct. The duct would vibrate when the air was on, and the corner was pretty sharp, which caused the duct corner to 'saw' its way through the cable's insulation over time. Took a while to isolate the problem to 'its between this box and this box' but was a pretty quick find after that :)