click or rattle on start-up when cold

I sometimes hear a click or rattle sound on start-up when cold. it lasts from 1 second to 2 or 3 seconds. never when warm. Any ideas on what this is? Seems to come from drive belt area. Not always there, has not gotten any different over time. I have seen question similar to mine posted on other sites for the SLK

Fred,
Yes, I have seen this same issue on other sites as well. My 2001 SLK230 has done the same thing since new... more apparent after sitting for a couple of days.

Dave