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.

hey you know I had a 95 Camaro that did that and it turned out to be the exhaust pipe hitting the cross member but only in certain situations , not all the time.
You might check that.

My 94 does the same thing, it is the fan hitting the shroud. Only in reverse too. I think my motor mounts may be worn out.

motor mounts are they hard to replace?? and are they expensive? is this a serious problem?

motor mounts are fairly cheap ($20-$30), it is a major problem because it can cause other problems, and they are faily easy to replace with the proper tools. engine houst, sockets and ratchets.
chris