My mother owns a 2004 2-Door Chevrolet Blazer with 4x4. Tonight, after she came home from work we noticed a noise starting and it sounded like a bit of a slow drone...then when she turned off the air something under the hood made a loud grinding noise, it would grind really loud and then quiet down a bit then start loud again and when she popped the hood and we looked around the part that I circled in the picture at the bottom of my post, (it's not of her engine, it's dark outside so it's hard to take a picture with a camera phone so I found one of an 2004 Blazer LS...the parts look the same so I figured I'd use it) and she turned off the vehicle then started it back up, and no weird noises or anything. Then she turned on the air...the noise kind of started and when she turned the air off again the grinding noise was louder. Now the part I circled in the diagram, as you can see has a belt that goes around it...and the circle part the belt goes around actually starts to move(which it didn't before she turned the air on/off) and that's what makes the horrible grinding noise...so if you could...please try to tell me what part that is and what you might think it could be.

Anyways, thank you in advance!

Here's the picture:
http://i52.tinypic.com/2d0f1hu.jpg

 

That's your AC compressor. The clutch and/or bearing has gone. Dont use it anymore until it's fixed or it will be much more expensive to fix.. a whole compressor and AC service instead of just the clutch and bearing.

See that electrical connector on the shiny can at the left edge of your pic? Disconnect that. Doing that will prevent the clutch from engaging, though that may not help much. Take it to shop asap

 

Thanks Petty! The more I looked into what you told me and watched a video or two of people with the problem, it's definitely that. Won't drive it tomorrow, but it will take it to the shop Tuesday. Will a 16 mile drive to the shop hurt it too bad, if the air conditioner is off?

 

If it's making grinding noises with the AC off and that plug disconnected, yes. The thing is -it seems like- the bearing is bad and it will ruin the front part of the compressor. Which mean new compressor.

The bright spot is that if the compressor itself had gone bad, you'd be replacing some expensive AC parts along with it.