One very old school technique ( and I mean OLD) is you take a long screwdriver and place it on the valve cover while the car is running. Then you place your ear at the handle part of the screwdriver. This provides you with a screwdriver stethescope which allows you to hear your valvetrain. As you slowly moving along the valve covers, you can zero in on the spot that is creating the noise. I did this and found out that the #4 intake rocker had slipped off the spring.
Some possible causes could be a collapsed lifter which is probably the simplest fix IMO. Another few issues may be a bent valve or running too thin of a certain type of oil. I've found that my 350 (after I chucked the factory 305 in my 80 malibu) did not like 30 weight oil once it warmed up. Once I put in a 40 weight oil, the engine made less noise and worked better down here in the south. Basically, a thicker oil would have better viscosity once heated and help your lifters and other valvetrain parts.
Another way to narrow down the guess work is to pop one valve cover at a time and start the car. Luckily, this is easy to do with chevy motors as they won't throw oil all over the place like mopars. I did this a few times without oil getting anywhere in the engine bay (adjusting rockers). The screwdriver method will help you so you know which valve cover to take off instead of taking off both valve covers.
I'm going to be tearing into my engine again soon because it had overheated at one point and has a very subtle knock. So I'm going to be investigating that. It sounds like a low deep knock and I'm hoping it's not in the bottom end of the engine.
