ok i have a 99 blazer zr2. when first starting truck its at about 40. when it warms up drops to about 20 and makes a clanking noise that comes and goes. im not exactly sure whats wrong with it. and pressure is fine while driving. so im replacing the main bearings changing oil and filter and cleaning oil pan screen. tell me what you think.

 

I think your throwing parts at it hoping to fix an unknown problem. what oil and filter are you using? its not uncommon for the valves to make noise, you need to isolate as best you can where the sound is coming from.

40+ cold and 20 hot is perfectly normal.

 

sounds like its coming from bottom end. i was thinking it might be bearings but at same time i think it might just be gunked up and screen blocked.