"Only" in neutral? It could be the clutch release bearing. With the motor off, tip the bike on it's side, with the clutch on the bottom. Hold it there about 5 minutes to flood the bearing with oil. Put it back on 4 tires for 5 minutes and then start the motor, and see if the sound changes.
Still there? It almost sounds like a exhaust leak. Take about a 3ft length of hose to your ear, the other around the motor and exhaust till you pinpoint exactly where the noise is from.
Have you tried doing what Dez started about tipping bike yet? Valves would be more like a ticking sound than whistling, but it’s something that should be looked at regardless as part of a maintenance program.