My 2005 Sable sometimes makes a whistling sound usually after I have driven it for 20 miles or more. Sounds like it's coming from somewhere around the intake. It only happens when the car is idling (in park or completely stopped at a stop sign). I've taken it to 3 mechanics including a Ford dealership. None of them could find anything wrong and were completely stumped. No error codes, no bad gaskets, no vacuum leaks, no exhaust or catalytic converter problems. Nothing. The car has 90,000 miles, has been very well maintained and runs like a champ (no power loss, smooth idle etc.). But that whistling is freaking me out. Sometimes it does it. Sometimes it don't. Any ideas?