Macrospilota
I have had indoor setups for many years. I have heard this often from softshells to snappers and all sorts of Emydid turtles. From my experience the sounds come and go, but often this is followed by what most identify as a respiratory infection. I personally try to make changes in the setup to ease the stress. Usually stressors come from cool drafts of vents, lack of adequate basking areas, time the basking lights are on, or dirty water. If you wait until there is a loss in appetite or lethargy then you may be in for a lot of TLC, time, and expense. I have found that the whistle will usually pass in a matter of days but some times it goes for months. I have seen what can happen so I now react as early as the signs may alert me. I am not saying that you should get antibiotics just watch them carefully. hope this helps, Scott