That sounds like it might be a Duo Valve problem.

Located next to the computer box on the right side of engine compartment.

Sometimes a good rap with a screwdriver handle will "fix" it. Kartek did a nice write up dissecting his and repairing it.

__________________
Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)