A brief rattle at the start might be the chains tightening up if the tensioners are getting a little sticky. It would be worth having an independent mechanic take a listen. That rattle could also be cold injectors (yep, we all got those!), a loose radiator fan (perhaps a cracked blade root? This has happened before) or a fuel pump that doesn't like the cold (not common).
If the tensioners are sticking it's probably gummed up with sludge, but you'd get a fair idea just by popping the oil filler cap and looking inside. If it's gummy up in there, the tensioners are likely full of it - and if it's clean up there, you might be ok. Tensioners do sometimes wear though, and it might be this that you're hearing. In this case, inspection is required and is not a simple task.
A second opinion is valuable. A third moreso, just to be sure. It's worth taking it to a diesel specialist, too - diesels are different beasties to petrol, in spite of the many similarities there are things like this that are perculiar to diesels.
Yes, my car is a 2009 diesel (359,000km now). My bike's petrol, so I'm familiar with both - although the bike is a whole lot quieter than the Nav!
Which reminds me, I have to do the plugs on my bike, and do a brake bleed ... oh the fun to be had!
