When I brake I hear a light whistle sound every time unless a step on the brake harder then the sound goes away. When I drive normally I do not hear anything (unless I start lightly braking). Not sure if this is a sign of a brake wear indicator.
I suspect a leaking brake servo booster diaphragm. My '76 Triumph TR7 did exactly the same thing, so I disconnected and plugged the vacuum hose.
The noise was gone, but it made for an extremely hard brake pedal. I don't recommend you do that, but you could do it as a short test to see if that's the source of the noise. Don't drive the vehicle, just depress the brake pedal while stationary.
It could also be as you suggested, the wear indicator. That sound is more of a screech - nails on chalkboard. Does it seem to come from behind the dashboard, or from one of the wheels?
It's a simple matter to remove the wheels and see if any of the wear indicator tab(s) is touching the brake rotor...