When the guitar is being played? What guitar? As you use an SM57 I should assume it is an electric one, recorded with the mic from the amp speaker. Electric guitar equipment is almost always more noisy than any other kind of equipment, so it seems likely the origin of the noise is the amp. That's natural and occurs on all records, use a noise gate to avoid this noise in silent passages; in loud passages, it should not be audible on the final record unless you put a lot of compressor on the track afterwards. (If you need so much compression, consider doing it with an FX pedal before the amp, that often sounds much better anyway.)
But then, you say you need to turn the interface's mic gains all the way up, that should not be necessary when miking an electric guitar amp!
If it is in fact an acoustic guitar: don't use dynamic microphones then. Not only do they have too low sensitivity (resulting in the need to crank up the preamp, causing noise level to explode), they also have a less than optimal frequency response for acoustic guitar. Even the cheapest condenser microphones usually give you much more sound clarity than generic dynamic mics, especially when not using a high-quality mic preamp, and a lot less noise.
