I too had a similar query when I got the 100-400, that lens make a fairly loud and audible clunk when the IS is activated.
The 24-105 is fairly quiet by comparison. The short buzzing / whizzing sound is most definitely the IS engaging.
16-35L, 24-70L II, 70-200L II, 100-400L, 50L, 85L II, 135L, 24TSE, 40, 100 macro, 600RT x 4
I have more photo gear than I need. The blame lies squarely with Canonrumors