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.
1DX, 5D3, 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, 1.4x III, 2x III, 600RT x 4
The grass is always greener when you crank up the saturation in photoshop