if ur miles are unknown on those heads, you need to pull the valave covers and check the conditions or springs, rockers, and even push rods. i had a jeep grand cherokee with the inline 6 that someone didnt do the routine oil change on and there was soooooooo much sludge build up and the lack of oil pressure caused one of the push rods to actually burn a hole through the rocker and then the one next to that was broken in half believe it or not. that knock was only slightly louder than yours and when i say slightly i mean not much but same thing happened when you revved it the knock got faster. so yeah id check that out. whatever it is, doesnt sound good. do you have any misfire codes or anything to set your check engine light on?
