Is wire arcing a potential danger?
Was the air handler running? Metal duct’s should be grounded but sometimes are not. 
I have seen metal trunk ducting build up large static charges. In one case it was about every 12 seconds there was a snap quite loud.  The charge was building up and discharging to a water pipe.

The white noise could be the close proximity of the cable to the duct so the charge is building up quickly and discharging. This will be the case only if the air handler is running. 

I would want to verify if it is static charges building that the duct gets grounded large static charges can hurt like &amp;$%#. 

If the charge is there without air flow I would be checking the electrical system (grounding, possible only ground the water pipe replaced with plastic) OR if there is a voltage on the cable.  The cable should be grounded at an inter system bonding point but if the main ground has failed this could have a floating voltage.