Tapping noise in top end

Was beginning to lose coolant and started overheating on hills on freeway. When I would get home could hear coolant boiling in recovery tank. Diagnosis: bad head gasket. So I replaced it with new head bolts, gasket set, valve stem seals as they were leaking badly in the middle two cylinders, (number two and number three) on exhaust side, and a minimal basic resurface head. When job was done it ran great, had better power, smooth idle, no problem. Then after a day or two there started a noise that still sounds like a bad hydraulic lifter, but of course this engine has no lifters. I have done two compression tests, results were average compression was about 145 with less than 5% difference between all cylinder's, checked valve adjustment three times, all to spec, checked timing chain and guides for any problems, and everything else I could think of, to no avail. Last detail, noise is usually not present at start up, but occasionally can be heard faintly, but as engine warms up the noise increases to level of your average lifter motor with lifter beginning to fail, but every so often it runs as quiet as ever and whether noise is present or not. It runs quite well, plenty of power smooth, responsive and surprisingly well for what it is and miles on the motor. I am really at a loss at this point, so any ideas as to what this may be would be greatly appreciated. Thanks in advance for your time.