it's an M45, and I've noticed something else. The grinding noise comes in as soon as third gear is engaged, regardless of power application, it does it as soon as the gear is engaged even with the clutch still down.

However, the shifter feels very loose in third, there's almost as much sideplay in gear as there is in neutral, changing gears is rather like stirring a bowl of porridge. I've discovered that if I push the shifter to the left, the noise goes away. Could I have a rod or something contacting where it shouldn't? And where do I look.

Not that this is the highest priority, I just took the distributor rotor off and it looks like someone used it for spotwelding practice. I won't know anything about the rest of it's problems till I sort out the ignition, I've already replaced the main pump and found a vacuam leak that had it running on 2 cylinders.
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Drive it like you hate it