It's normal to hear a slight whirring-like noise when IS is active. Also, IS does not disengage immediately after a shot is taken; there's about a 3-second delay after you fully release the shutter before it shuts down. IS will stay active as long as autofocus is active or the shutter release is half-pressed. What you're describing sounds totally normal.

I just purchased my first Canon 'L' series lens, an IS model to boot.  This evening when I was storing my gear after a day of shooting I noticed for the first time that when I have the IS turned on and autofocus it makes a rather interesting noise, even after I take the shot.  The closest thing I can compare this to is a ball rolling along.  Is this normal (I'm assuming it is) or is it a bad thing? Anyone know?

When I first got my 100-400 the IS would make a thumping noise and jerk the image around.  That went away very quickly and now all I get is the ball rolling noise you describe.  I think there may be a wearing in period for these things.