Yes the Eclipse does rattle !
I mentioned this in my Antec 900 review and people were like no it doesnt....
I use to wake up at 4am in morning and have to tap my eclipse on the side to kill the rattle.
Only thing I would advise is like the above, u gotta open it up and track down that rattle for me it was the fan on the front and the foam, when I turned off that fan no such rattle, but I also noticed the front case isnt so sturdy/firm and that caused it partialy.
hopefully your rattle isnt random like mine, therefore making it difficult to track down ! but as a last resort take the front off and the turn the fans off see if it goes...
Never know could just be a screw loose or something not screwed in properly.