Hey!
I was wondering if anyone else had had this problem. I've noticed a rattle coming from what I believe is the front driver side it's hard to tell almost sounds closer to the firewall but it doesn't make the noise until I'm pressing on the breaks and put it into gear. If it's in park or neutral I can't hear it. It also makes it under low rpm not noticeable at highway speeds but I've already replaced the pcv valve and checked the transfer case shield also checked the cat and that heat shield the cat sometimes hitting it with a mallet I could hear a bb rattle so I bought new cat and took it in to have it put and and exhaust shop couldn't get it to make the noise so they didn't want to put it on. I'm confused as to what it could be sense ot sounds more dominant on driver side any help would be appreciated

 

I also have a 1999 4.3L 4X4 and I am chasing what sounds like the identical same problem as your truck. I initially thought it had something to do with the power brake booster and changed the check valve and and also disabled the booster completely and the noise is still there. Also changed the PCV valve but that did not help. I checked all vacuum lines under dash and they appear OK, Am now looking onto the EGR system. I may have a leak in the line that connects to the exhaust coming up from the back of the engine(close to the firewall) going over the intake manifold to the EGR valve on the front of the engine. I was also told that since mine had an engine rebuild about two years ago(before I owned it) that they may have put gaskets in betweeen the heads and exhaust manifolds and there should be no gaskets there. If they did install gaskets it will cause a leak?????.
Also mine has now gotten progressivly worse!! once the engine gets up to temp the noise is there all the time. I can only hear it when I am stopped and sometimes when I depress the brake it comes and goes?????
Will keep you posted on what i discover.