Classic/Antique Car Repair: lifter noise on Cadillac, cadillac eldorado, push rods

Question
Hi, I know that this car is a little newer than your usual.  I have a 1984 Cadillac eldorado factory convertible with a 4.1 litre that I just Purchased in North Carolina.  I drove it 700 miles back to Mass. It ran real good for the first 200 miles and then I noticed that  the drivers side lifters were making a lot of noise.  I checked the oil and I had plenty.  About 400 miles later I stopped for gas and the noise was gone. Now the noise is back louder than ever after driving at highway speeds for about 50 miles.  Can they be adjusted?  

Answer
JOHN;

I would check the springs-lifters-push rods,etc. they might just need a adj. than I would  drain the oil and filter , put in new oil  and add a can of STP to it with the new oil and filter.  if the adj. or you find nothing bad than the lifters are shot or the cam is shot or both.