Well, one good thing about silhouettes is that they usually involve a bright light source, so you can use a lower ISO speed. This should help in decreasing noise. Of course, you also want to make sure your shutter speed is fast enough to ‘underexpose’ so that the silhouette is black and not gray.
So:
Lower your ISO,
Raise your shutter speed and or aperture.
Then check your image to make sure that your subject is black, not overexposed.