pressure plate besides other looseness in the drivetrain. Try this, start the and car back down the driveway and then do the same slow release that you describe back up the driveway. This time the clutch system will not be near as warmed up as when you pull in from being out and about with the car. If the chatter dissapears (or greatly reduces) then the issue is most likely with the clutch. Usually with clutch chatter the springs get weak in the pressure plate. Also oil on the friction disk can also cause chatter issues. There are also springs in the friction disk that get weak from use but that usually effects the grab feeling of the clutch, weak springs here usually cause the clutch to grab to quick with light pedal release.
