My sisters Passat has a 'chatter' noise when setting off and turning or when near full lock moving slowly. A garage diagnosed it as a failing tensioner and replaced it along with the belt.
Howver, it didn't sure it. I had a look and the noise is the from the PAS pump! The pump is slowing down causing the belt to spin on the pulley. So for £40 she got a 2nd hand pump thinking the pump may be seizing.
Today I fitted said pump, drained all of the old fluid, filled with fresh, ran for a few mins then drained and refilled. All now nice and green but it still doing the same thing!
It's as if something is restricting the high pressure side flow and causing the pump to slow up momentarily.
Is it the rack that is causing this? Or maybe the filter in the reservoir? But this was pushing fluid up when draining and turning the wheels lock to lock. (Obviously nut under the same pressure though as the engine was running...)