What Kenneth said. I just noticed the same problem with my Mustang recently. When the clutch is disengaged, I get a nice whirring sound. Dern throw out bearing. Have the new clutch and all..just no time to put it in.
I have a 2001 Jeep Wrangler with manual transmission. When I have the clutch pushed in, there is no noise. When I have it in neutral and let the clutch out, there is a fairly loud whistling noise. Push the clutch back in, it goes away, it doesn't stop immediately, but takes a second or two to go away, sounds like something is spinning and has to come to a stop.
The clutch seems normal otherwise, not hard to push in, doesn't pop out of gear, etc.