Vacuum sound on passenger side of engine

On my 99 V6 Passat, there is a noise which comes on at times which sounds like a vacuum cleaner. It is on the passenger side of engine and will come on even after car is off. The dealer said it could be the secondary water pump. Any ideas?

Sounds like the discharge hose has come off of the secondary air injection pump. The pump is located in the right front of the engine compartment. When the hose is off it sounds just like a vacuum cleaner. However, the only time the pump should run is when the car is started cold, for about 30 seconds. It's purpose is to inject air into the exhaust when the car is below a certain temp, to help the catalytic converter lower emissions. It should NOT be running when the car is off.