A loud whistle or "air hissing" noise is heard when the heater-A/C system is in use.

In vehicles equipped with in-cabin Microfilters for the heather and A/C system it is possible that the Microfilters cover is not attached properly causing an air leak. To check this remove the glove box or other trim finishers to access the Microfilters cover and check fit. If the cover seem loose or improperly installed, reattached properly.