Strange rattle & slight jolting sensation..

I recently noticed a strange sound from my Focus when I first start driving it. Only thing I can think of sounding similar was when the heat shield on my previous mondeo came loose, a metallic rattling / vibrating kind of sound. That sound occurred constantly though, whilst this is intermittent & only lasts a short time (30 secs, etc).

However tonight I picked the car up from a friends where it had been since yesterday morning & this noise occurred as soon as I started it & lasted for several miles - it was also the loudest & most constant that I've heard it (could easily be heard at around 30mph over engine noise, with the windows closed) but seemed to stop once the engine got warm. A couple of times there was also a very slight "jolt" as if something wasn't quite fully engaged, but then slipping into place. (crap explanations I know, but can't think how else to describe it!!)

Near home I stopped for fuel & it was fine when I started again, so I wondered if it could just be something as simple as the TDCi not liking the cold weather? It has seemed abit harder to start lately since it got colder.

I did recently had a cambelt change done & am hoping it's not related to that in any way - I've no reason to suspect the guy who did it of !Removed! anything up, but you never know.....?

Just a bit worried as I drive about 100 miles a day to work & back, & don't fancy breaking down on the motorway at this time of year (or anytime of year, come to that!)

Cheers if anyone can offer any advice.

Vas