On Engine shut down A/C off-on my 2003 Pontiac Aztek the A/C system begins to whistle. System...

[Peavey12]

On Engine shut down A/C off-on my 2003 Pontiac Aztek the A/C system begins to whistle. System...

...performs OK.? The A/C system works OK, however after the engine has been shut down the system begins to whisle loudly

 

[deltech]

Its just the pressures equalizing in the ac lines. normal operation

 

[Mr. Badwrench]

It's normal! The sound is just pressure equalization in the system.

 

[John K]

Your System While Running Is Two Sided
One Sude Is Suction Or Low Pressure
The Other Side Is Under High Pressure
When You Shut Down The Pressure Equalizes From One Side To The Other
There Is Usually A Certain Amount Of Noise Invovled As It Does This
Why You Are Getting A Loud Whistle Is A Puzzle
I Suggest Taking A Piece Oh Hose Such As A Heater Or Garden Hose A Couple Feet Long
While The Noise Is Present Use The Hose As A Stethescope And Pinpoint The Location
At That Time You Will Be Able To More Properly Diagnose The Repair
Since Your System Is Working It Is Not Losing Freon
Any Repair May Be Costly Even If The Part Is Cheap Due To The Need To Evacuate And Recharge The System
It May Be A Simple Harmonic Vibration Somewhere In A Line That May Be Cured With Insulation Or What Is Called Dum-dum Tape