That lens, by the way, always has a very soft electronic sound, even when "IS" turned off.
I can only hear it in a quiet room when I put it to my ears.
I believe that's something about "floating glass elements" that are engaged when powered on. You can hear the elements clunk about inside when the lens is powered off ( that's also normal).
But, you indicated it's a much louder sound and you just noticed, it so I'm guessing it's "PRE-AF ON"...
If it's simply the soft noise of the lens then that is normal.