I'm currently using a brand new Canon 100mm f/2.8L macro lens. I noticed that when IS is enabled and I press the button mid-air (no shoot), I hear a little noise. Is it the image stabilizer? With IS disabled, the lens is absolutely silent. I'm asking because I have another lens with IS (18-55mm standard kit lens), and when IS is enabled I hear absolute nothing.
Yes, that's what it is. Different lenses use slightly different methods for implementing image stabilization, and it's more audible in some than others. Among other things, longer lenses need more motion to compensate. If you listen very closely, I bet you can hear it on the 18-55mm as well.