I have a 2001 Blazer 2dr Rear Wheel Drive... Whenever I hit bumps, I can hear a knocking noise in the passenger side near the firewall. The ball joints on that side have been replaced. I simply cannot figure out what it is. Any thoughts?

 

Control arm bushing?

 

I had that same problem with my '02 Blazer. I would hear the noise on small bumps and when applying the brakes, right on the passenger side like you have. Since years ago I had a similar problem and replacing the badshocks solved the problem back then,I replacedminebut the annoying knock persisted. Eventually I found out the problem was the idler arm. I replaced it and the knocking stopped, almost completely. I had warped front rotors so I replaced them too and now it no longer knocks. I hope this helps.

 

Check the torsion bar mount also. Had one go out on a 92 jimmy and it sounded like hell. Replaced the entire front end before someone mentioned that mount.