I've had a Comptech SC'd S2k for years now, and just recently it started making a rattle noise on cold start. It goes away in less than half a second, but it's pretty loud. It only happens if the car has been sitting around for a few days without being driven.

Anyone know what might be the cause? There's no obvious reason from visual inspection...

 

If nothing is loose, bearing maybe? Next time you're gonna have the car sit, take off the supercharger belt. That way, you can rule out the supercharger if the noise is still there.