When i start my '99 blazer there is a loud clunk from the rear, what could this be?

 

Nothing should be moving in the rear of a Blazer when it is started, other than the fuel pump pumping gas to pressurize the fuel lines. So following that line of reasoning, could it be possible that your fuel line(s) is not properly anchored to the frame and actually jerks/moves when it gets pressurized? I can't think of anything else that it could be?!?

I wonder though, could you think it's coming from the rear, but it's really coming from the front? Maybe a broken motor mount....?????

 

Maybe damaged or interfering exhaust components...?

 

I had a new muffler and such put on a while back and recently it started the same thing. I had the better half start the truck while I was back under there and found the exhaust tapped on the shackle(or spring)when it first started. Just bent the hanger in a little and no problemo since. Something to try.