Noisy autofocus on Sigma contemporary

I have a Sigma 150-600 contemporary telephoto with Nikon mount that occasionally becomes quite noisy; a whirring noise, when constantly tracking moving birds.  I believe this is the hypersonic autofocus motor making this noise.  The lens still focuses but the noise is a concern to me.  The problem is intermittent.  I've had the lens for a year and it did this initially and has done so periodically over the one year plus that I've owned it.  I mentioned this to Adorama, where I purchased it, and they basically didn't think it was an issue and blew me off.  However, as it does this more and more, I am concerned.  It does have a four year warranty.  Has anyone else had this problem?

I haven't noticed it being particularly noisy, David, but now I'll be listening for it.

I don't recall hearing noise during autofocus, but there is a noise which I think comes from the vibration compensation function.

Well, I could be wrong about the HSM and it could be the OS.  I was using it this morning.  The noises are concurrent with focus and OS use.  Lately, I have been using my Nikon 600 f4 and  generally I use it with the VR off.  But I just turned the VR on the other day when it was unusually dark due to rainy weather.  I noticed it was noisy and the noise was similar but different to the Sigma lens.  So Allen, you could be correct and this might be the Sigma OS.  But is it normal?