2008 Honda Civic rattling noise

First remove the belt from the crankshaft pulley, or just remove it completely, then run the engine to verify the noise is gone. With the engine stopped, feel each pulley to see if one is loose and wobbly, or makes a crunching or binding sound. They should all spin freely too.

Normally when an idler pulley has a bad bearing, that pulley will be turned or tipped a little. That will make the belt slide across that one or the pulley right after it, as it goes around it, and that will set up a squeal.