Engine knocking up top

My blazer starts good. After start-up, the engine sounds quiet but as it starts warming up, a knocking/ticking sound starts to appear. When cold, the oil pressure reads about 60 - 70 and after warm up/ driving, the pressure reads from 20 - 38 at idle. Of course the pressure will go up when I rev the engine or taking off. I opened the driver side valve cover and it seemed to be clean with no sludge but I still took out the rods and sprayed through them with the chem tool carburator cleaner. One of the lifters somehow popped up a little after I sprayed it with the cleaner, that I had to turn the engine so I can re-seat the rod. It was the first lifter towards the front of the vehicle. Since this side wasn't that dirty, I chose not to mess with the passenger side valves as there's too many hoses and AC stuff in the way.
The thing is from what I've researched online, it seems that, if the lifters or rods were bad, the noise should exist no matter the engine temperature, or exist when cold and go away when warm. When I listen around the engine, with a dowel or a long piece of metal, no noise from the valve covers or the top of the engine. I can't really use the dowel or the wire to listen to the front part of the engine due to all the belts, so I just lean in and listen and it almost sounds like it's coming from the area where the two bigger pulleys are at. Biggest pulley being the center one and also the other one to the right of it. (Sorry for my description as I don't really know which is which other than to guess that the biggest pulley would be the one attached to the motor itself).
I recently bought this vehicle with 194000 miles on it. Changed the oil and filter right the same day and saw it was fairly dark oil with a little sludge but no metal shavings. I put in Mobil 1 high mileage 5w-30 and an engine repair additive. One week later about 77 miles into it, I changed the oil and filter again. I used fram filter both times. Still not seeing/ hearing a difference. The engine is not losing power or dying on me but I'm concerned this may lead to bigger problems if I don't nip it.
Once again folks, I ask for your help in getting this figured out. Symptom is engine is quiet when cold, starts knocking sound after warm, regardless of rpm, idle or run. And knocking speed follows the rpm as well. My vehicle is a 1996 Chevy S10 Blazer; V6; 4.3 liter; Vortec; 4X4

Thank you

OK, Last month my check engine light came on. I took the vehicle to a Chev. Dealership and they told me that it misfired and needed a major tune up at a cost of 800.00. They called and said both engine mounts were broken at an additional expense of 600.00 and that I should service the transmission. Total bill to pick up this vehicle worth 3000.00 was $2000.00, I drove away to experience tthe brake peddle shaking. Turned around after 7 miles and was told that the hub and bearing were out, and that the radiator was cracked. After these additional repairs, te car was vibrating (1 week later) and I was told that it needed rotors turned and brake pads. I have always had the car serviced at 3000 miles and transmission serviced at 50000 miles and no REAL problems. I have had the car back for one month now and a 100 miles from home last night I was on a 3 lane highway doing about 74MPH in cruise control when the car started a violent shake, it seemed to loose power and by the time I got it pulled over it died. I looked under the vehicle and did not see anything leaking or no smell. I tried to restart the vehicle; it started immediately but was shaking and I could hear a knocking under the hood. A few minutes later I tried the same thing, with the same results. I had the car towed ($250) to the dealership that did original repairs, but they are closed until Monday. Any ideas or suggestions? Woman neening advise!

Last winter I started to get a loud engine knock when starting up. As time progressed it got worse. I thought it sounded like an exhaust leak but when I got out to listen it sounded like parts were going to come flying through the engine. It does not do it all the time. It lasts from 10-15 seconds to 10-15 minutes but only on start up. When it does stop it stops all at once, not gradually. Sometimes it does it again after a short time sitting. Other times it takes sitting overnight to do it again. One mechanic told me it was the cam shaft pulley. That is tight and does not seem like the problem. My regular mechanic said it is the lifters.

Okay. What do I have to do to repair the problem?