If they all are like mine I don't think it matters if it's flush mounted. I took it apart removed the center pin, inserted a smaller drill bit
through it and it didn't go all the way
through, it's sealed about 3/4 way up the center pin channel, all the airflow is coming trough the side hole. I stopped up the hole on the center pin at the bottom when I put it back together and it didn't make a difference.
Wound up drilling the side airflow to 1/16" and it's awesome now, I can adjust with the allen screw from completely shut off to a pretty wide open draw or anywhere in between and no whistle. Totally happy with this thing now.