I have a 2011 Nissan Xterra Pro 4X. A whistling noise started coming from my AC last Summer and it is gradually getting louder. I tried changing the air flow to my heater and it stops. Back to AC and it picks up the whistling noise again. Is this coming from the compressor? Any feedback would be appreciated.

Switching to heat pretty much rules out anything inside the blower or HVAC Unit, since on Nissan HVAC's the incoming air always flows through the A/C Evap but only passes through the heater core on a heat setting. There isn't much in the A/C system under the hood which can whistle besides the compressor and fan, and if the A/C works correctly, that pretty much leaves out partial blockages or air in the plumbing. Sticking your head under the hood while an assistant cycles the A/C on and off should make it obvious whether the noise is from the fan or compressor. If it seems like the latter, putting a stethoscope on the compressor should verify it.