Rattling Sound hear when starting engine

[GigaCrusher]

Hi,

I own a Civic ES for almost 5 years and has been serving me well until lately when I am hearing (on occasion) a rattling sound (much like that of a rattle snake) for about 30 secs before it died down by itself. There appears no other symptoms when driving off. I wonder if this is a sign of problems to come. Please advice as I am hitting 82,000km and have yet change my timing belt but will do so soon.

Thanks very much in advance for any kind bro's advice.