Whistling sound when HVAC is on

[Mike64]

I have a 2012 Sierra, that has had 4 condensers replaced by the dealer shop.

It was replaced in July 2015, and the AC has gone out yet again.

When it starts to fail, there is a whistling noise while moving. I was able to work it out to get an extended warranty from GMC.

if anybody else has experienced this would really appreciate any input