A transformer will hum whether it is loaded or not. Possibly hum
louder under load, but they will hum irregardless.
A/C .vs. DC for transmission, it's easier to convert AC than DC for
long distances. Current is the killer when it comes to transmission.
The more current you are sending, the more losses there are in the
lines. DC conversion to higher voltages takes a few steps, one of
them basically being turning it to a pulsating DC (or ac) and running
it through a transformer type device to boost it, then re rectifying
it back to DC....and subsequently can have more power lossess in the
process than AC conversion which requires that you simply run it
through a transformer... not to mention the equipment probably will be
more expensive as well. Look at the price of electricity, even 1 amp
of 'lost power' can turn into a lot of $$$ over days/ weeks / years,
etc etc when you are running at voltages of several hundred KV..
Another issue is, that some DC conversion equipment works in a vaccuum
(ie tubes) and over 10Kv in a vaccuum can generate X rays, whether
wanted or not...
Back in Tesla's days, X rays probably were not even thought of, or
worried about, by todays standards though..... anyways, another
'issue' to worry about with power conversion / transmission.
With A/C you can send the power literally hundreds of miles, with DC
you'd need several substations along the path.