So, my rebuilt 401 has run great since first startup. Having been completely balanced it is also the smoothest engine I have ever had. After having put a couple hundred miles on it, while idling in my garage, it started making a "hellatious" clattering noise, causing me to shut it down. After collecting my thoughts, I restarted it, ran nice, then the noise again. I pulled the intake, rocker covers, etc., and removed and inspected the rocker shafts (rebuilt by Rocker Arm Specialists) the pushrods, removed the lifters and examined the FACES and cam lobes, all looked great, returned lifters to their original bores. I removed the timing chain cover and examined the Cloyes double roller timing chain and gears, all looked brand new. I reassembled all and started up, ran well and then the clattering noise again! I pulled the pan next, super clean with no metal in pan, clean pump screen, all rod and main bearing torques spot on. Viewed from below, all bores look excellent, no contact marks, or signs of mishap. I will be reassembling as soon as I get a pan gasket, but I'm sure upon restarting it would run well for a bit then begin clattering. I believe that I must have a lifter that is collapsing on me, what do you guys think?? I'm not sure how to determine for sure which lifter(s) are the problem, thinking of getting a set from a known good supplier, perhaps TA. I welcome hearing your thoughts!