come on bump...

That does not sound like a noisy preamp to me.  Usually preamp noise is white noise, like a hiss.  This sounds like some some kind of electrical interference buzzing. 

First, make sure any gear connected to the interface is using the same electrical circuit.  That may clear up the noise.  If not, it could be that your studio has some RF noise that is more noticeable than usual given the high gain required by the SM7b.  Try swapping the cable. If that doesn't work, try using a quad shield cable.  I have RF problems in my studio and find that quad shield cables work great to reduce the buzz (except for single coil pick up guitars, but that is another issue).  Both Mogami and Canare make high quality quad cable.  You can solder your own, or you can find guys on eBay to make them for you for a very reasonable cost.