Low Oil Pressure and Knocking

[ChevMech]

I have a 2003 Z71 with a 5.3. In the past month or so, the oil pressure has dropped. It is sitting on either side of 40 p.s.i. where it would run further to the right in the past. As far as the knocking goes, there is no rhyme or reason to when it starts knocking. It sounds like you would be hitting the oil pan with a hammer. I was told it was probably a lifter making the noise, so I doped up the oil with transmission fluid (an old old old remedy for what I understand) and it didn't work. After I changed the oil, I put a lifter cleaner in and it has done no good. I'm going to go out on a limb and say it isn't a lifter. The one time it made the noise that wasn't in the middle of the road, the sound was much louder when I was laying underneath than when I was listening to the top. Does anyone have any idea what I could be up against?