Rocker Noise

[dencon]

My buddy's 68 pick up with a 350 startd to get rocker arm noise the other day. But they did not make the noise all of the time. Only after he had been driving for a wile. So this past weekend we check to see if they needed adjustment. They were loose so we adjusted them all up. The truck sounded fine after that, for the weekend. Then driving to work, the noise came back after about 10 or so miles on the freeway. So I told him to watch the oil pressure an the way home. He called me and told me the oil pressure was fine all the way but the noise would come back after a few miles down the freeway. Never has the performance of the engine change in any way when this happenes.
I am thinking that his lifters may be collapsing and are shot. What do you guys think? Any input?
Thanks
Matt