Just finally got this thing running pretty good, but im experiencing a weird clunking noise. On start up everything is fine, idles great. Put the blazer in reverse or drive and I get this horrible clunking noise like metal on metal. Sounds like its coming from the passenger side. Thought it might be my exhaust but everything seems pretty sturdy. Only happens sometimes doesnt always do it. Its a 91 auto 2wd 4.3 tbi. Only 68k Im hoping I can get many more miles out of her.

Anyone know what it could be so I can start checking things out?