Sounds of air or low growl under dash when on start up?

Some times when I first start my truck I hear a sound like "air" or an " air purge" in the dash.
It only makes the sound for 5 -10 seconds at most. And only when you first start the truck.
It only does it may 1 out of 50 times I start the truck.

The sound is on the passenger side in side the dash. The two time I heard this I had the A/C turned ON.
Also I was parked on a slight hill with the passenger side a little bit higher on a slope.

Could the sound be antifreeze filling the heater core? Maybe because it was on a slope the heater core drained out some. But it really doesn't sound like water. Coolant level is fine. Completely full.

It sound like air. Or maybe like low growl. Maybe even describe it as a grinding sound.
But it stops with in 10 seconds of starting?