There are two things you gotta do when you drop tune a guitar like that:
1 - you have up your string guage to .011s (an .009 tuned down to a "C" is likely gonna buzz with any reasonable action)
2 - you have to COMPLETELY re-setup your guitar. You will not only have to tweak the truss rod to get the neck relief set correctly, but you will have to raise/lower the bridge saddles and adjust the intonation.
You should get the store you bought the guitar from to do the set up - they owe you one, I'd wager, seeing as how it's a brand new guitar. Just take it in to them and tell them exactly what tuning you want and say that we recommend a heavier guage strings. Then mention that you want the relief set properly and the guitar intonated. Once they've set it up properly, you can use that as a base line to start learning how to set up your own guitar, but if you think you can just say "I've got a string buzz when I down tune to this weird tuning and I spent a lot of money on this guitar and tell me how to fix it!" and we can post a message that gives you the magic bullet, you're fooling yourself. No guitar is self maintaining - they all need to be adjusted appropriately when you change things. Necks move, bridges need adjusting, the intonation has to be set, etc.
The bottom line is that it's clear to me from your previous posts that you do not have any idea how to set up your guitar and your guitar needs to be set up. Have a pro do the set up, ask him a bunch of questions, and buy a good book on guitar set up (like any of Dan Erlewine's) if you want to be able to do it in the future.
my 2¢
