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?

SOURCE: 1997 Mercury Sable GS

Two possible sensors could be affecting it like the TPS or MAF sensor. The Throttle Position Sensor (TPS) might have a bad spot where the ECM won't get signal properly and causes the RPM to fluctuate

SOURCE: rough idle

Mercury had a recall on 3.0 duratech motors in 2000 and 2001. If you get a fuel smell at startup then you might have a bad lower intake gasket, it could cause a miss if seal is cracked. Article No. 01-4-3 you can get it on AllData.

Also, check the PC rubber L piece that runs from the PVC valve to under the throttle control - it could be busted.

Make sure ignition wires are not cracked. If you haven't tuned it up in over 30-40K consider doing that.

SOURCE: 1989 Mercury Sable GS 3.0L runs rough and idles down and stalls

Sounds like you might have a stuck open EGR valve. Is the check engine light on. I would replace the fuel filter if it hasn't been replaced, also check the air filter.