Bit of a strange one, ever since I have owned my Focus Mk2 1.8tdci estate there had been what I can only describe as a metallic ringing sound coming from the engine under acceleration only. The car drives fine, pulls like a train, I had it remapped and it really goes now but this noise is still there. Its just had a new clutch and DMF so I know its not that. Is this a common thing? It almost sounds like the blades of the turbo are touching the turbine housing but there are no marks, turbo fine and there is no oil in the pipes, Im quite sure the is not boost leak either.
I noticed the noise when I took it for a test drive and the mechanic I bought the car from said they all do it?
Any Ideas

 

Had exactly the same problem after my clutch and master cylinder were replaced. The engine also used to appear to growl a bit when accelerating. Turned out that it was because the exhaust had been removed but when it was replaced one of the exhaust fittings (brackets?) was rubbing against something. five minute job by garage realigning the fitting fixed both issues.