1996 Toyota Camry Need Toyota Camry guy here- Several Quest

I just bought a 96 camry with the 2.2. The A/C compressor bearings are bad, so when I looked at it, I really couldn't hear the engine. Today I took the belt off temporarily just to hear the engine by itself, and I can hear a knocking/rapping noise coming from somewhere in the block. It sounds kind of like spark knock, but with a hard tapping. When I rev the engine, it seems like it gets worse for a 100 or 200 rpm, but then it seems to go away or at least be covered up by the rest of the engine noise. I used a screwdriver to try and locate the problem, and I can hear it most severely at idle at the bottom of the block where the oil pan bolts on. I think I have heard rods with low oil pressure in My old toyota rap before, and they sound quite different, like a deep thunking sound that I could hear from inside the car. This is a high pitched rap/rattle, can only be heard under the hood. But I have listened before at the oil pan with a screwdriver and not heard this noise. Could you tell me what it is? I also have a couple of other questions: For this vehicle, when the engine is totally cold and at idle, if I pull the radiator cap off I get small burst of coolant coming out. Any other vehicle I have seen the coolant isn't even circulating and is under no pressure until it warms up. The antifreeze looks okay, and I don't see any large bubbles. Should I do one of those UV dye tests to see if there is an issue with exhaust leaking into the cooling system?It seems to me that something is wrong there, and perhaps the noise and that are somehow related. And I have one of question, I hope someone can answer it. If I decide to keep this damn thing, will engines from any other year camry interchange with the 96? I looked on car-part. Com and its only showing me 96's when I do a search. That makes me suspect that other years are not interchangeable. But if I could just change the needed components over, wouldn't there be some way to fit a 92-95 engine in there? Knowing these things would help me decide whether to keep it or not. Hey, I am giving you what I can as a donation, I hope its enough to get some decent answers. Thanks. Al