When I was changing my rocker cover gasket, I felt one, and only one rocker was "loose", like it could pivot on its axis. The only motor I know anything about is air cooled VW, and loose rocker is normal. That is what this felt like. So is this loose rocker the culprit in that "bad lifter rattle" that I sometimes here? Is it loose because the lifter does not hold its rod firmly against the rocker? And if thats the case, does that mean that particular valve is not opening as far as it should an/ or is it opening late? Is it detrimental to the health of my motor?

 

I would remove the rocker and then the pushrod to see what's going on.
The pushrod might be bent.