It's hard to tell sometimes if your loosing oil pressure and that shows itself as valve train noise in its early stages. The valve train needs a lot of oil.
That being said there is one thing that happened to these rocker arms that's worth mentioning. There is a steel insert inside the aluminum rocker arm that would get loose just a little bit and make a lot of noise. It's easy to check with the rocker shaft on the bench. Put a little pocket screwdriver into the lube hole for the pushrod and gently try to turn the insert in the rocker arm. If it's a loose/noisy rocker arm you will be able to turn it.
If you find a loose one, post it. You need to know about left and right rocker arms.
It could be the lifters, or it could be a cylinder sleeve starting to loosen. The lifters make a noise similar to a diesel engine at idle. While this sound might annoy you, the motor can run for a very long time with this occurring.
The louder knocking sound is a cylinder sleeve, and is the beginning of the end for your engine block.