The chainguard would make this noise not from rubbing the derailler cage, or from pedaling (since pedaling doesn't interact with the chainguard at all, only turning the wheel does). Rather, chainguards can be slightly loose and wiggle around in their place and this can make noise. But it's still an off-chance.
Repacking a hub is relatively simple. For the 105 rear hub you'd need 15mm cone wrenches, I think. Park Tool has a useful guide:
http://www.parktool.com/repair/readhowto.asp?id=105
Also, since the sound comes when you're pedaling and when you're not, you know it's not a problem with the freehub.