Here's why:
* Blend Door Actuator: This small motor controls the blend door that mixes hot and cold air. When the AC is off, the blend door may be shifting to a different position. If the actuator is worn out, sticking, or has a failing gear, the resulting movement can create a loud whistling or whining sound. The noise is often more pronounced when the AC *turns off* because that's when the actuator is actively changing the blend door position.
Other less likely but possible causes include:
* Vacuum Leaks: The system that controls the blend doors relies on vacuum. A leak in the vacuum lines can cause erratic operation of the blend door actuator and lead to noises.
* Loose components: While less likely to cause a *whistling* sound specifically, something loose within the HVAC system could rub and make noise when the AC is switched off.
Troubleshooting Steps (Do at your own risk):
1. Listen Carefully: Try to pinpoint the location of the noise. This can help narrow down the potential problem.
2. Check the Blend Door Actuator: This often requires some disassembly to access the actuator, usually located behind the dashboard. If you're not mechanically inclined, this is best left to a mechanic. A visually inspecting for obvious damage or a sticky mechanism is a start.
3. Check Vacuum Lines: If you're comfortable working on your vehicle, visually inspect the vacuum lines connected to the HVAC system for any cracks, breaks, or disconnections.
Recommendation:
Since getting to the blend door actuator often involves dashboard disassembly, it's generally recommended to take your 2004 Buick Rendezvous to a qualified mechanic for diagnosis and repair. Attempting to fix this yourself without the proper knowledge and tools can lead to further damage. The cost of repair will likely be less than the cost of fixing additional damage caused by an improper repair attempt.
