I think the crackling was the sound of static charge, picked up by the aircraft as a whole, arcing across (flashing across) the non-conductive elevator bearings to the elevator, from which it escaped via the static wicks as normal.
That's my guess.
But yes it could have been the other way around i.e. static charge picked up by the elevator arcing across its bearings to the airframe, from which it got dissipated via the static wicks on the flaps, and the (somewhat less effective) static wicks on the ailerons.
The COM2 antenna is a lot closer to the elevator than the COM1 antenna, hence...
what about the 'only detectable when there's carrier' part
No idea. These radios are damn complicated digitally synthesised jobs.