This happens mostly during colder weather. If its more like a whinning noise right after you start the engine or you stop then seems to fade as you accelerate and get the engine warmed up, it's your power steering pump. Check the fluid level when the engine is warm, fill up if on minimum. Start your engine, get it to operating temp, rev up then turn the wheel to right till you feel a slight resistance, turn left, repeat at least 5 times each, check the level again. it should quiet down if level is only issue.