it could actually be your air pump. there is a plug for it on the right hand side of the front wheel well, disconnect it and try the car again, if there is no noise then your air pump is shot. I would pull your passenger side valve cover forst to check your chain and tensioner. If it is the the air pump just keep it disconnected till you get to your mech then decide if it is worth the fix.
2005 320 CDI
2006.5 VW Jetta TDI
1991 560SEL (179000 Sold)
1972 280SEL 4.5 ('The Lead Sled' 320000 miles when sold.)
1972 220D (225000 when sold)