loose flywheel ?

[Bart97]

I have a '97 F250 HD 5 spd. Recently I started getting a noise like my flywheel ringing. It sounds like someone holding a screwdriver handle against the flywheel while it's spinning. It happens only after it warms up, and it doesn't do it at full throttle or down shifting but it does it at idle and at cruse.Could the flywheel be slightly loose on the crank and vibration causes it to ring ? I can't think of anything else that would ring like that. Not a duel mass and everything looks OK.