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.