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.
5D3, 6D, 600D, RX100
16-35L, 24-70L II, 70-200L II, 100-400L, 50L, 85L II, 135L, 24TSE, 40, 100 macro, 18-55 II, 55-250 II, 600RT x 4
I come here to learn something new, not to learn how bad my gear is - I know that already ;-)!