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

Are you sure its not a clogged cabin filter?

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra

Mpolli - that's a great suggestion, I'm ordering some other parts this week and am due a set of cabin filters. I'll let you know if the noise goes when they're changed.

Thanks,

Peter