Knock at high rpm's???
 

I notice my car makes a very bad noise! Remember those plastic machine guns you'd buy as a kid that made like a clicking noise as you'd pull the trigger? My car makes this sound when you rev it up and let off, this is when it makes the knock, say around 4-5k. I almost think it comes from the passenger side valve cover/head area. Some say it could be a lifter of rocker arm. Any ideas? I'm getting ready to take it apart?

Could very well be part of the valvetrain. Change the oil and examine the oil for any metal pieces and pull the valve covers and have a look.