Some lenses with image stabilization make a low vibration/humming sound. This is the vibration reduction motor in action. If your lens has an on/off for the stabilization, try turning it off and see if the noise continues.
As well, many lenses with stabilizers incorporate one or more 'floating' elements. These elements are loose when the stabilizer is not activated, therefore you may hear them lightly moving around/shaking when the lens is not turned on. This is normal.
Different manufacturers use different names for their image stabilization technology. Keep an eye out for these names and abbreviations:
Canon – Image Stabilization (IS)
Nikon – Vibration Reduction (VR)
Sony - Optical SteadyShot (OSS)
Sigma – Optical Stabilization (OS)
Tamron – Vibration Compensation (VC)
Panasonic/Lumix - Optical Image Stabilization (OIS)
Pentax – Shake Reduction (SR)