ok, I had this noise before and after I changed Lifter, pushrods and Cam. All came from the same shop which is http://www.buyperformanceparts.com/

I will check for an exhaust leak but I really can not imaging that it can sound that mechanical.

Furthermore I will check the Lifter again with pressing a hammer handle on the rockers while engine is running in idle.

Maybe it is a really good advise to have a look at 273 rockers. But I don't want them adjustable, keep the hydraulic lifter. Do these use cup-cup pushrods?