You probably only just noticed this when you were in a quiet environment - you focus, that noise ends, and then you're left with a secondary whirring noise coming from the lens. The 18-200, as a small light lens, is possibly the most obvious one for this.

 There is nothing wrong with your lens - it's the optical stabiliser. To keep up with framing, and to avoid lag when taking the picture, the stabiliser will continue to operate for a period after the focus has been set, even if you have released the shutter. If you're working in manual focus mode and anticipating a very fleeting subject, it's worth lightly pressing the shutter to AF position to make sure OS is running, though the delay is minimal.