Vacuum leak in Focus cabin

I am driving a 2006 Focus wagon. After 10-15 minutes running, I begin hearing a whistling which sounds like an air leak (actually, it sounds like a very small police whistle). It can be heard loudest under the
dash on the driver's side, and its pitch rises and falls with the RPMs of the engine. It continues to grow louder with further driving.
I've had it to two dealers three different times and nobody seems to be able to reproduce it. Any ideas?
Thanks.
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snipped-for-privacy@aacps.org wrote:

Are you asking for help in finding it yourself?
If so, locate the vacuum line into cabin and disconnect, plugging the port to engine.
If it stops, reconnect and note if the sound changes with changing heater controls. Then locate lines to heater box and check what happns when they are disconnected and plugged.
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