I had the clutch release bearing renewed - the old one was loose and dry, and this has eliminated the spinning-down noise, and greatly reduced the rumble when the engine is running in neutral. But the vibration is still there. However, I happened to notice that it got worse after I loosened the fan belt, so I'm now suspecting the alternator or water pump bearing, or the viscous coupling on the fan. Will check this out when I get time. I have a spare water pump, alternator and viscous coupling so if my suspicions are correct, should be sorted soon.