2001 Chevy Blazer, has loud noises coming fom the engine on start up.

"Rod knocking?" If you're certain it has a knock, then a couple simpler checks can tell you how bad it is - pull plugs one at a time, if knock decreases you've found the cylinder affected. If knock or tick occurs at same rate as engine RPM, it's the lower end, if it's half the engine RPM it's cam or valvetrain related. Check you don't have an exhaust leak too, which can make a pretty good ticking. You won't really know how bad it is without tearing down motor. Only you can decide if it's worth keeping or sending to that big scrapyard in the sky.