Hi Krosen
Does this occur if you create a new session (or a quick drive session), and then go to that spot on Healesville?
Are you in CAB (realistic) mode, or DCC (easy) mode?
It may be the safety valves on the steam locos, which produce a loud 'hiss' sound when the pressure gets too high in the boiler. In DCC mode, this will be automatic (and may be a little more often). CAB mode should be a little easier to control with some practice, but of course is a more complex control system.
Regards