Hi All! New to the site and this is my first post but felt I might be able to help or at least share my experience....
I've got 2004 ST170, same problem, odd rattle at idle, which disappears when I either put my foot on the clutch or drive off. I have changed my clutch to a sprung loaded plate and single mass, light weight fly (Thanks to TurboSport).
I was told about the vibrations and extra noise from the single mass fly but don't have either of those issues. I was told my issue was the idler pulley bolt working loose which I replaced and it cured the problem. There was also times when the car would idle fine then as either the fan cut in or I turned on the A/C the noise would start. New idler bolt...done....
On another note, lightweight fly has really helped the car rev freely and I've gained an extra 85 miles of range in fuel economy!