i know mine isnt the heat sheild its a very noticeable knock coming from the back or bottom of the engine. what else would cause a loud knock and low to no oil pressure?

 

mine did the same thing you said, in my case it was the main bearing was spun when my high pressure oil line blew, and oil would leak past, causeing almost no pressure, when the truck had a cold start i had pressure untill the oil warmed up(the warm it gets the thinner) and once it was warmed up i had 0 psi at idle, driving i had 7psi, then i put SAE50 weight racing oil in it(i recomend this as EXTREME sercumstance and try not to in cold weather because the oil is SUPER thick, like malasass)and eventually my pressure would drop to 0 t an idle but around 15-20 psi driving, however being such thick oil it was so hard to start in the cold that the starter ended up chipping teeth of the flexplate(flywheel) and i spent a month starting my truck by using a ratchet on the cracnkshaft to turn it and get good teeth for the starter to bite(it sucked), finially after 2500 of pushing and off-road abuse it blew on one slight little rev to get thru some slushy snow on a gravel road, my crankshaft exploded, when the shop replaced my engine, 9v battery sized hunks of cast iron fell out the oil pan, when an engine is knocking due to damage bearing there are only 2 fixs, both very exspensive, put a new engine in or replace the damaged bearing, either way its a pain and costly, i have heard engines ping and tap because(believe it or not) some people dont know they have to get their oil changed and it gets dramatically low, i have heard, but not seen, of engines with bad plugs and low oil, impropper gas, and i even think a clogged injector i have heard(dont know for sure on this one)but those to my knowledge are pings, serve damaged bearings and knock(with a pressure issue like yours) are almost unmistakable, i got an engine for mine with 135k on it for 565(incluging shipping) off ebay that was checked and tested good(only thing is someone removed the CPI from the intake and sold it seperatly so my old on was use), i have it in the truck now, shop charged 675 for engine swap, mine lasted 2500 miles knocking though, try to drive very gently as far as taking off goes, dont gun it, take your rpms up slow and gentle, even if it means being the jerk on the road thats going to slow lol
ive also heard that some 4.3ls had a problem of the suction hose on the oil pump dropping off into the bottum of the pan, cause the lost oil psi, but from what you say it deffinatly sounds like mine did, sry

 

lol, man that sucks! sorry I havent replied in a while, I havent been back on here since I thought that Id sell my blazer. but, ive finally decided that ill be putting a new transmission in, because BLAZERS RULE!! Ive also came to the conclusion that the knocking is coming from the torque converter/transmission.
by the way, I do play guitar and Randy Rhoads is the reason why I play, my biggest influence and hero! awesome you listen to his music too! (god bless him)