Mechanical rattling sort of noise that gets faster in parallel with the engine revs (not wheel rotation speed).
Starts off quite quiet and get's progressivley louder as the situation deteriorates.
Ended up blowing a hole in the engine block but didn't go with a bang, more a quiet pop and instand loss of power.
I can only thank the lord that it doesn't effect the driveshafts when it does go, simply because I'd be underneath a lorry right now if it did.