In cab whistle with a/c on

[Jim Lynch]

This is driving me nuts! With the a/c or defrost on, I hear a funky high pitch noise between 1500-2000 rpms. Turn the air off, fan on, it’s gone. Air on, it’s back. You can hear it if your on the passenger side with hood open. Air is cold, just the noise is obnoxious. Somewhere I read I could be a couple ounces light in Freon. Orifice? Truck is ‘13 extended LT 4x4 with HD cooling.

thanks