Im not 100% where I would put this so here it is. Within the past 3 months or so my 2003 Chevy Trailblazer 4.2L has been experiencing some issues. Occasionally on ignition and sometimes randomly while driving the engine fan gets extremely loud. Like ridiculously loud and it just appears for awhile then goes back to being quiet again. Another thing is that my engine has a knocking sound that comes from it occasionally. Both of these issues are random but it seems they happen at least once a day. Thanks for the help!