Brake boost vacuum leak?

Yes, we had this same issue with my wife's 2001 S40. The short formed black plastic tube that runs from the junction connector down to the pump, had cracked and slipped off the pump connector. Car was suddenly idling very rough when sitting at a stop. When we parked the car and shut it off, it was buzzing like an electric alarm clock. I taped the cracked end on the black plastic tube and slid it back onto the pump, problems immediately vanished. Unfortunately though, electrical tape is only a temp fix. I'm searching for a new tube, auto parts stores don't carry the plastic type line (piping) needed. They have rubber vacuum line but it will not slide fully onto the self lock fittings made for the plastic line. Take Care

Oh, and also, when the line came off, brakes were virtually non-existent .