Chiphead hit the nail on the head! I had a 1956 just like you are talking about and it did just like you said. What the problem is, the bore the lifter is in is getting worn a little bit and when the engoine is hot the oil viscosity is not heavy enough to keep the lifter pumped up. I knew which lifters it was that were leaking down, I put in new lifters and it was the same old thing. I cured the problem by inserting a new solid lifter in that bore, adjusted it for the proper clearance and that was the last time it made the noise.
