I have 93 blazer 4.3 and when i go to first start it up it has this knocking noise like something is hittin something and it only does it for a while n then i don't hear it again but lately it has been making the noise only when i reverse...does anyone know what could possibly be my problem? it's a rather annoying sound, thanks.

Flywheel may be loss or cracked. Check to ensure that all the torque converter bolts are tight on the flywheel. These will sometimes back off causing this issue.