I'm not saying a "broken rocker arm". I'm saying the ball of the rocker is not good anymore. I've had this happen before. And when you pull them all off to inspect them, they all look fine....put them back on, and the sound persists. Replace them one by one, and viola, the sound goes away with one of them being replaced.

If you had a collapsed lifter, like Tommy said, the rocker would be loose, and no way to tighten it up.

Is your valvetrain adjustable? (You said it's stock, which would indicate it's not....buuut....) I would like to listen to it sometime, but unfortunately, I am training this morning, and then this afternoon, I have a lunch date with a new girl. Sooo....Yeah...I MIGHT be able to drop by after around 3 or 4. PM me you addy and a phone. I'll text you if I can make it around then. That way, I can make a better diagnosis.

Who did the rebuild? Were the heads rebuilt also? Did they just do the short block? Or the long block?