engine clatter

Hi people I am posting this for a friend of mine in the hope that someone can help.His car is a 2007 focus 1.8tdci with 80k on the clock
(ex lex lease car with full service history) the problem is the car developed a loud clatter from the top end I have heard it and it is loud its coming from the front half of the engine Strange but you can get rid of the noise by holding car at about 2000rpm but outside that it comes back in again, There is a slight misfire at about 12-1500rpm with no load on the engine and he tells me that the car is down on power( I suspect the two are related) The car is sitting in a garage at the moment waiting for him to make a decision on what to do The garage I think is stumped as to what it is but have said it could be:- Little end bearing ( but why would that be intermittent) broken valve spring ( possibly but again why intermittent) bend valve ( car hasnt been overrevved why would valve bend)
They have also dimissed faulty injector "as they dont sound like that" and have dismissed faulty hydraulic tappet (which was my prime suspect) as its got shimmed followers Wha I can say is that the clatter sounds to me like its timed the same as the cam and is harsh and there are no fault lights and oil pressure light goes out instantly
Has any body had a similar experience with this engine or any good advice to give before we get into large expense as the garage suggestion is to put in secondhand engine
Many thanks Alan
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