Sounds like the harmonic balancer might have let go which is not uncommon on the M57.
The black smoke may have been the rubber if there was slippage between the inner and outer parts.

Try to identify ,exactly what side of engine ,noise is coming from!Could be a broken turbo shaft.No air=black smoke (too rich).
When a turbo breaks ,it will send the propeller down to exhaust pipe and compressor to intercooler !

Where was the black smoke coming from? Tail pipes or under the bonnet (hood)?

If the smoke came from under the bonnet, then jdd is probably on the money - the harmonic balancer (the large composite pulley on the front of the crankshaft, that drives the v-belts) has probably failed. A few other folk here have had a similar failure with similar symptoms.

Turbo or harmonic balancer aren't going to stop making black smoke and not likely noise either. Wouldn't be consistent with getting it home and starting it up and everything is normal. But just in case, what did it smell like. fuel smoke and rubber smoke smell totally different.

Not a rubber smell. More oil fuel smell. I thought it came from engine bay and exhaust but can't be 100% sure all happened so fast.


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Maybe the CCV?

Did you change the oil separator?
The old one (fleece) of the M57 can be replaced by the one of the newer M57N (cyclone - 11127799366).

If it was running fine, no noise and no smoke after you got it home, I would look for oil on the floor now that it has been sitting and trace the source if there is some. If not, I would start it up with the hood up and let it idle and see if it still runs normally.

If you haven't checked for trouble codes I would do that first.

I agree,Harmonic balancer,obviously doesnt make any sense,plus we hardly had any issues with M57 engine,only M47!
I made the turbo possibillity on his OP first post!
Unfortunately ,the OP hasnt provided much information and the fact the engine now doesnt smoke or make noise,I still dont know if OP has actually driven it.
Black smoke only intermittently could be easy to diagnose!
AIR SUPPLY
1.Some foreign ,matter(like a leaf) blocking air filter and then dislodged (M57 has a long cylindrical filter with a small air feed!
2.Intermittent AFM issue
3.Intermittent blockage os EGR system
4.Intermittent intercooler and pipe blockage
5.Vaccuum pipes intermittently blocked and not opening wastegate.(Vac pipes are only 4mm inside diametre and as they heat up ,they tend to close slightly!On old vac pipes,they close completely and need to be replaced!)

FUEL SYSTEM
1.Intermittent injector blockage(instead of spraying ,it leaks) that cleared up.
2.Common rail fuel sensor or MAP intermittent issue.

Now the fact OP reported a knocking noise that has since cleared (has it actually cleared when he drives the car or has he driven it?),suggests the possibility that there could have been an exhaust blockage,like a broken catalytic converter loosing parts out to rear of exhaust system..

All these possibilities are very common ,not just on the M57 ,but until we get some more feedback from OP we really cant say!

No smoke or noise now since the recovery. I haven't driven it far. Probably 5-6 miles since but the engine when revved sounds all good. It appears abeit I have not taken it on a long journey to have cleared. As a precautionary measure I've had it serviced (oil/filter/airfilter) and all seems well. The knocking noise was so severe when it initially happened I assumed no recovery was possible


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I would try starting the short drive with the engine oil and coolant at operating temp, radio, lights, and air conditioning on and see if the symptom was due to overheating.