Getting an occasional noise from the steering. Like a humming but it lasts for just 1 second when turning at lower speeds (although at higher speeds it may not be audible). Feels mechanical, like the pump is struggling or having to work extra hard. It's just a hum, nothing more. No creaks and no change in steering characteristics. Had a look for a dipstick to see if it was low but no sign. My memory suggests these cars might not have one. Any ideas? It's a A3 TDi 170 Quattro. Cheers.