Had a few hours to spare on Monday evening to have a closer look at what's causing the rattling, decided to take off the alternator belt to eliminate at alternator and water pump. Started the car and the noise was immediately worse, checked the crank pulley and could get a very very slight bit of movement. Took off the starter motor, locked the flywheel with a screwdriver and managed to get an adjustable spanner onto the bolt and got it tightened up ever so slightly. Checking the pulley by hand again and couldn't get any feeling of rotation.
Put all the pieces back together, started it up an the noise had disappeared. Drove it to work the next day and it felt very sluggish, but no rattling noise, had the timing gun in the car so took it out for a run at lunchtime to get it up to temperature, set the timing to 12° btdc, the drive home was much more livelier.
Then was on wedding car duty for my brother's wedding, just over an hour's drive to the venue, didn't miss a beat. He then took it off after the ceremony for photos, all arrived back safe and sound, although I forgot to tell him to ignore the fuel gauge saying it was empty on a half tank!
**But***....there's always a but....
Since tightening the crank pulley bolt, I have a bit of a whirring noise when it's idling, that goes away when the clutch is pressed.
