OS Noise: Sigma 105mm F2.8 EX DG MARCO OS Lens

Hi,

I just bought a Sigma 105mm F2.8 EX DG MARCO OS Lens and it works very well (after 20 minutes of testing with it).   BUT, the OS motor makes a fairly loud vibration noise during focus.  Is this normal? If so, I can live with it for the price, but if not, I'd like to know ASAP in case there's a problem down the line.

Thanks!