I noticed, last winter,when the car had not been used for a few days that there was a "rattle" for just a couple of seconds on starting. I put it down to maybe the timing chain adjuster sticking and then freeing off. No hint of it when being used often and in the summer months.
I came to use it a few days ago after it had been sat for a couple of weeks and the start-up rattle manifested itself again. This time I gave it closer attention. I revved up to over 3000 rpm and the rattle reappeared with some urgency. Went over 5000 rpm and it suddenly disappeared and has not come back.
I have a feeling that there is a loose nut or bolt in a cowling which occasionally comes into contact with a pulley, or such like and then gets flung into a tight space somewhere and then gets ejected back into wherever the pulley is, when everything has a chance to cool right down.
Everything else to do with the engine, apart from an annoying minor water leak, is OK and I have done over 5,000 miles since the rattle first occurred!
Anyone had anything similar? Previous cars:
S-Max 2007-2013 (only diesel I've had, good car but expensive when diesely bits go wrong, so what's the point?)
Galaxy 2001-2007
Mondeo Estate 1997-2001
Sierra Estate 1993-1997
Uno Turbo 1987 -1993
Fiesta 1984 - 1987
Fiat 127 1982 - 1984

I'm wondering if it has to do with the chain tensioner of the cam chain. It may have been a bit "stuck" and the high revving loosened it up again. Although the original AJV-8 has some issues with the cam chain, our versions should have the improved version without issues. MY 2010 5.0 SC Galway green and sand interior!!
Have the Faultmate MSV2 Extreme to be tinkering with the settings etc. !!

Does it have hydraulic tappets??, probably does..... if it does, it's because the oil will drain from them if left standing for a while and it will take a some time for the pressure to build up after you start it, I wouldn't rev it up if I were you, just drive it normally until it goes away... Pete
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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand Sold
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Sold
2002 L322 Vogue 4.4 V8 Epson Green (Fatty Offroader) Sold
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Definitely sounded like a loose nut/bolt/screw. I was not daft enough to rev it high from cold. I did have a belt changed last year (summer) not long after I had purchased the vehicle.
I will keep a weather eye/ear out to see if there are any other unusual noises that would indicate if water pump pulley or alternator have fallen off  Previous cars:
S-Max 2007-2013 (only diesel I've had, good car but expensive when diesely bits go wrong, so what's the point?)
Galaxy 2001-2007
Mondeo Estate 1997-2001
Sierra Estate 1993-1997
Uno Turbo 1987 -1993
Fiesta 1984 - 1987
Fiat 127 1982 - 1984

I was going to say tappets too, especially as it goes when the revs are raised, that would be the oil pressure going up and re-filling the tappets. Might be an idea to have a listen under the bonnet on start up to try source the general location of annoyance before you rip the engine apart !! Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

Rattle was originally on start-up. Last time it happened though, engine was warmed up and I dabbed the throttle to take it above 3000 rpm. The rattle returned with more urgency but then disappeared for good, when I took the engine above 5000. I won't know for sure until mid- November, when the car will have sat doing nothing for a fortnight, whilst I am at work! Previous cars:
S-Max 2007-2013 (only diesel I've had, good car but expensive when diesely bits go wrong, so what's the point?)
Galaxy 2001-2007
Mondeo Estate 1997-2001
Sierra Estate 1993-1997
Uno Turbo 1987 -1993
Fiesta 1984 - 1987
Fiat 127 1982 - 1984