I have a Trane heat pump system that is 2 years old. The air handler is a TWE037E.
When the system is running, there is a noticeable "hiss" noise coming from the lower right-hand area of the air handler where the condensation drain pipes are attached.
Here is a pic of a Trane AH (not mine, but looks very similar):
The red arrow points to a small, square-shaped opening between the two drain outlets. The hiss noise is caused by air suction into the AH through that opening.
Is this opening meant to allow fresh, outside air into the system? And does this outside air pass through the filter?