As long as there is no grinding or ratcheting sound, I would say that it should be normal. In a quiet environment, I can hear almost all my lenses to make some sort of sound while focusing.
BTW, have you identified whether the sound is made by the IS or the AF module?
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