Hissing noise

As others have said it sounds like probably a normal converter cooling fan. But that should get louder not quieter when you turn on more loads. Unless maybe your generator is undersized or feeding too may loads. In that case I imagine as the trailer is taking more power than the generator can provide the fan will slow down. 

if you turn the power off does it stop? 

the way your describing it where you turn lights on, it gets quieter which doesn't really make sense for the converter fan as it should be off if you are using now power and only start up when the unit starts getting warm.  every rv I have owned if your power usage is low it is silent then as you use more power the fan will start to keep the converter cool.  so adding a light should make it louder if it is the converter fan unless something is wrong with that itself

From your description It sounds like the noise is most likely coming from the converter fan, if so,  it’s not a problem, normal.

You can confirm this (if your unit has a battery in place) by unplugging from the generator, then the lights will still work normally, but as you turn them on and off there will be no noise coming from the breaker panel area.

Could be multi-level battery charger running, the noise is probably the fan.