Break up some dry ice so that you have fist sized chunks.  Take one of the pieces in a gloved hand and press it hard against the metal plate.  The heat from the plate will make the dry ice sublimate very rapidly, causing a large amount of vapor between the plate and the ice.  By pressing the ice to the plate, the vapor has trouble escaping and creates a high pitched ghostly whine. 

If you do this demonstration for extended periods of time, you will need to warm the plate in hot water occasionally to keep the demo squealing well.