Quote:
Here's what Is stumping me, it only seems to happen when the interior ventilation controls are set to the defog setting and the interior fan is at least on setting 1. Changing the fans speed doesn't seem to make a difference. I have only tested with AC off so I think AC can be ruled out.


On GMs, the air con runs in defog (defrost) mode (even if the AC is set to off) and the fan is forced to speed 1.

The only high pressure thang associated with this is the ac compressor. If your description of high pressure air venting is correct, you probably have a failing expansion valve and what you hear is either it opening way too late or the ac system is venting freon due to over pressure.