* Vacuum Leak Impacting Idle: The idle speed is controlled by a delicate balance of air entering the engine. A vacuum leak introduces unwanted air, disrupting this balance. When in Park, the engine's load is minimal. A small vacuum leak, while insignificant under load (like when driving), can be enough to disrupt the idle speed in Park, potentially causing a stall. The whistling sound suggests air escaping somewhere in the system.
* Transmission's Involvement (Indirect): The transmission itself is unlikely to be the *direct* cause of the whistling or stall in this scenario. However, components *near* the transmission, such as vacuum lines that run through that area, could be the source of the problem. These lines often control various engine functions, including the idle.
* PCV System Possibility: While less likely to directly cause a stall at idle, a severely malfunctioning PCV system can create a vacuum leak, contributing to the whistling and potentially influencing the idle stability. A faulty PCV valve can allow excessive pressure to build up and create vacuum leaks.
* Why Not in Drive?: When in Drive, the engine is under load. This added load compensates for the slight vacuum leak. The engine has more work to do, and the additional airflow from the leak becomes less significant relative to the overall air demand.
Possible Culprits and Next Steps:
1. Inspect Vacuum Lines: Carefully inspect all vacuum lines near the transmission and throttle body for cracks, leaks, or loose connections. Pay close attention to the whistling sound's location to pinpoint the leak's source.
2. Check the Idle Air Control (IAC) Valve: This valve regulates air intake at idle. A dirty or faulty IAC valve can lead to erratic idle and stalling. It might also create a whistling sound if it's malfunctioning.
3. Examine the PCV Valve: Check the PCV valve for proper operation. It should be easy to test; a simple test involves removing it and seeing if there's resistance when blowing through it.
4. Throttle Body Inspection: The throttle body is also a common area for vacuum leaks. Inspect the gasket and the throttle body itself for any cracks or damage.
5. Professional Diagnosis: If you're unable to identify the problem, take your vehicle to a qualified mechanic. They have the tools and expertise to diagnose and repair vacuum leaks and other potential issues accurately.
Important Note: Ignoring a vacuum leak can lead to further engine problems. Addressing this issue promptly is recommended.
