W210 vent noise

[pwells]

Car is 1999 E430.

I suspect part of the vacuum circuit for the air handler controls has a leak as there is a hissing sound coming through the vents. There sound is quieter when the recirculation button is pressed. The noise is there when the AC is off and the AC works well so I don't suspect low freon.

Does anyone have any pointers on where to start diagnosing this?

Thanks in advance,

Peter