Hoping guys can shed some light on this. My newish Cummins built motor has had multiple irregular noises since getting it back from the shop and I've slowly gotten rid of them except for the last one which could be described as a lifter tapping in an old small block, the noise im chasing is rpm dependent and changes consistently with the rpm, it is a one to one noise. One rotation of the motor equals one tick. The noise is a clack or a smack like you'd hear if pulling apart two steel items that were stuck together hydrodynamical and starts 5 seconds after start up as the oil pressure passes 30 psi or so. I have tried many different oils and tried adding zinc. Power driven diesels oil makes the noise the loudest, it makes the noise more audible the same way adding zinc does. Lubrication engineers 8400 oil made the noise the quietest. Im not interested bashing PDD, theres nothing wrong with their product, its been tested. Oil samples all tested good with no abnormal amounts of metals. I replaced the cam gear for another reason, cp3 and its gear neither of which effected this noise. The entire fuel system has been checked thoroughly and eliminated as the source. I have the engine torn down for a second time, cam/lifters out, valve train torn apart, im micing parts and everything so far looks good. The plunger for the oil pressure regulator has a wear pattern on one side. Does this plunger move consistently with rpm? I can not find a specific operation description of the pressure regulator, but I understand how its supposed to work. Is it possible that with every rotation of the pump it forces the regulator open to bypass pressure? Pressure is 55-60 cold idle, 50psi hot idle, so im not concerned about pressure but I do have to find this noise. As you know once a noise gets stuck in your head its all you hear until fixed. I tow heavy daily and cover around 100 miles daily, I do not want something to fail in the middle of the country. The oil pan is coming off and the transmission is being pulled to get to the rear cam bushing and to check all associated hardware bolted to the crank. Thanks.