tarquin, sounds like my car, just as RPMs increase the noise increases. gets louder and faster.
Rallybob: yes i have hydraulics and they are the origional set to this head, so they're used. i took a old junk valve cover and cut the top off so i could bolt it on the head and see what the valve train is doing, and keep the oil in the motor at the same time. me or dad cannot find a "weak lifter". when it gets warm you can hear one tapping but can't exactly pin point it. it sounds like the cam gear is smacking the front of the head and its hard to hear other things in the motor, and we're thinking maybe that the cam smacking the front of the head is transferring all the way through the back of the cam, making us think that when we put the stethescope on the motor that it was coming from the number 4 cylinder, when its really the cam. we tore this head all the way down and soaked it in carb acid and did a valve job on it. i'm using valvoline 10W-30 and running about 2 bars of oil pressure at an idle of 1100-1200.