2002 Chevy Blazer loud noise

Maybe belt old an cracked and/or worn belt tensioner. Try and isolate noise to and area or pully

Is it a click then a few seconds and another click? Watch the A/C compressor and see if it is cycling with the clicks. When you turn most newer vehicles to defrost the A/C also cycles to dry the air and prevent it from fogging the windows.

If that isn't it then it takes a bit more work. If you determine that it is not the A/C then use a 3/8" bar to move the tensioner and release the belt from under the top idler. With the belt loose start the engine. Don't run it over 30-40 seconds. Listen for the click. Is it gone? If yes then start turning each pulley and listen/feel for a sticking spot, grinding noise or a very loose bearing. They can be difficult to track down so check close.