I have an 05 STi and when I have it at idle, it makes a certain noise... i can't really describe it... Maybe one can say it sounds metallic. Its not a ticking noise, it is a constant humming noise.
Then, when depressing the clutch, that noise goes away and then a weird alienish sound comes. It sounds like the movies when the UFO comes in for a landing. When I depress the clutch, the RPM's lower as well. If this helps, this started occurring after I drove through a flash flood.