So today I went to turn on the defroster as my front window was fogging up. There was a loud BANG from under the hood. Like, it sounded like a rod went through the block, I'm not joking... it was that loud.

I popped the hood and everything seems to be there. I know the A/C compressor can kick on during the defrost cycle, but I find it hard to believe it is that loud while engaging.

Has anyone else experienced this?

 

Compressor "slugging" sounds like that. It happens when a large enough glob of condensed liquid refrigerent passes through the compressor. Some vehicles do it worse than others and an overcharged system makes it more likely to occur. If you're sure that the charge is OK, there's not all that much more that you can do to prevent it. You could try turning the compressor on at idle speed the first time each day. That might reduce the noise compared to when the engine and compressor are turning faster.

 

Its the compressor engaging for a few seconds. Its supposed to do that to keep all the internal parts of the A/C oiled during the winter months. It is a big drag on the motor so that shock is transmitted through the serpentine belt then the rest of the motor.