Without having more infromation, there are a few things that I can think of right now that could cause a noise.
Bad power steering pump (check valve or fluid low)
wore out steering rack
wore out CV joints
wore out ball joints
wore out wheel bearings.
wore out stabilizer bushing
and I am sure there are plenty of more reasons. If you need more information directly related to your vehicle, start with telling us what vehicle you have and what noise you hear when.