Hello. I need a bit of advice from users of Sigma lenses with optical stabilisation. I have just recieved a new Sigma 120 - 400 OS and my first impressions are very good. Everything works fine but even though the OS function appears to work near as damnit as advertised it makes a very faint screeching sound, like a short circuit. as it engages and again as it disengages. It is worse when disengaging if the camera is pointing down. Is this normal or is it something to worry about? I used to have a Nikkor lens with VR and it didn't make anything like this sound when using the VR function.

Thanks

Phil