You might pull the dizzy & preoil it with the 5/16" hex shaft & see what you see for oiling. Not sure if a faulty filter (or bad brand) can exacerbate drainback but it'd be a cheap thing to try. Is your preload OK. I wouldn't sweat a few seconds of clattering but I would have to checkout the inadequate oiling on the rockers. I'm wondering if a thinner oil would build psi faster. Nothing definitive but here's a BTT for ya