Camera Labs

I have been looking around online but haven't determined yet whether there's something wrong with my lens. I just bought the 18-200mm f3.5-6.3 DC OS for my Canon bodies and I've noticed that I can hear the whirr from the stabilisation mechanism always. Whether the OS switch is on or off I can always hear it as long as my camera is on and it's constant, not just when I'm taking or preparing a shot. That seems really wrong to me, and wasteful, although through the viewfinder it does look like the image is only actually stabilized when OS is set to ON. Does anyone else have this lense that can confirm whether this is correct operation?
My other stabilised lens, which is a canon 70-300, distinctly only powers the stabilization mechanism when it's selected on and when the camera is showing metering info in the viewfinder.
I bought the lens at a big store and quite frankly am not confident that the answer they will give me will be informed or unbiased.
Thanks in advance.

<UPDATE> I queried Sigma directly and they explained this is normal. The OS mechanism is used even when OS is selected off to keep the lens centred. In their words "This is normal of the lens as this lens does not have an internal OS lock.". There's more to their explanation but basically what it comes down to is that the lens is behaving as designed.

Hello grosporina, and welcome to the friendly Camera Labs forum!
If you look through the viewfinder without half-pressing the trigger and you can see no stab-effect, than the OS is off as it should be.
I had a stabilized Sigma once and the OS was off when not activated through the trigger.
Still, if you can hear a sound from the lens when it is not focussing and not stabilizing something is wrong and you should get a replacement!

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Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews, My Pictures, My Photography Blog
D800+assorted lenses

Thanks, that's what I was afraid of.

I took the lens back to the store and checked another brand new lens right out of the box, it does the same thing. Difficult to detect in a store with the background noise but I could tell it was also whining _always_ when the camera body was on. So it seems that it's normal behaviour for the lens.
I'm very surprised but I'll just keep an ear on it and hope that if it's harbinger of worse things to come that they happen within the 5 year warranty.