I have had the problem before & found it was the one way valve in the clutch cover for the oil circuit, the valve was leaking over night & allowing the oil to drain back into the sump over night, first start from cold & the motor would rattle until the motor got oil pressure then the noise went away.

I did a full explanation with photo's of the problem on the forum a fews year back, I am going out now & will find it for you later.

Hi lads
i bought a bike 3 weeks ago and engine sounded spot on, bike only done 4,900 miles 2011 xt660x bike was kept in a worm garage. anyway i have had a loud noise plastic/rattle when i start the bike up from cold. its last 20 seconds or so. its about 2 or 3c here so is cold and my bike is my garage, but is as cold as the outside temp.i have been trying to find out what the problem is for the last two weeks, i have asked kev and so on, even ordered a new clutch to put in this week as the noise was not right and loud. anyway after reading your problem with and you saying it was only happening when it was very cold (i did not think it would be that at all) i put my bike in the kitchen of my house so would be nice and worm when i start it the next day, just to see if it made any difference at all, thought i may as well try as it cost nothing to give it a go.started the bike this morning NO noise at all ? not jumping up and down yet but think it was the cold and that's it. going to leave bike in the kitchen for the next 2 days without starting it. if when i start it the noise has gone, then it was only the cold. very odd but at least its not a real problem. will let you all know if the noise has gone in two days time.

2nd time I have ever heard of this.