24-105L Owners, tell me this is normal

Hi,

Using a 24-105L on an new 5DIII.  Lens snaps right into focus, but there is a motor "whirring"  sound right after focus and then a mechanical soft click. maybe less than 1/2 a second after focus.  Focus never changes.  It's definitely the is the IS motor as there is no noise when I turn it off.  Not sure how IS works so perhaps this is normal.  Can any owners tell me is this is normal or the experience the same thing?

thanks