Did they find anything other than gunk when they pulled the valve covers? A broken spring or bent pushrod should've been easy to see. But, being that you said it drove correctly before the oil change means you're on the right path. Add some more seafoam with your fresh oil and filter change and run it some more. I'd probably put on several hundred miles this go around with some good higher rpm pulls when it's not making any noise.
It's frustrating, belive me (daughter's nightmare lifter). It took some time and finessing, but it's been quiet as a mouse now for many miles. I will say that once a lifter gets gunked up, they are more prone to relapses with dirty oil. You got this man! Show it who's boss.....gently at first of course.
