My wife started her 03, 1800 Sunday morning to go on a ride and when it started there was a very noticable engine noise. It wasn't really a knocking noise, more of a rattle. She shut it off right away and then I told her to restart it. It made the same noise for about 5 seconds then faded away. She then rode the bike and it ran fine. It has done this once before, both times after not running for several days. Has anyone else had this happen? What might it be? The bike has about 30,000 miles on it.