I am getting a humming sound in my boat TV, new installation, maybe
someone has solved this before.

I have a standard Sony 19" LCD TV, and a automobile 12 VDC DVD player.
They are connected to each other via a cable with 3 RCA jacks (left,
right, video)

When I plug the TV into shore power, everything works perfect. Great
sound & picture.

When I plug the TV into 12 VDC (via an 12vdc to 18vdc step up
transformer), I get a light humming sound in the TV, picture is still
great.

As I go to debug this I notice:
1) Moving wires, has no effect on hum. (neither louder or softer)
2) If I power off DVD player only, hum remains.
3) If I unplug DVD player (from 12vdc), hum disappears. (even with RCA
connected)
4) If I unplug the RCA cables, hum dissappears

So clearly the DVD is the source of the hum (not the 12-18vdc step up
transformer), or is it ?

Any thoughts ?
Dan