40 engine noise mystery & compression down one cylinder

[L78CHEVELLE]

I have a ’40 221 flathead engine. Last weekend after a 35 mile trip the motor started knocking and the noise sounded like a valve pecking. Here is where the confusion starts. I was in a rural area and I had to try and make it home with the motor knocking. The engine was a little down on power. I drove the car about 2 miles and the noise started to go away. Within 5 miles the noise was gone, but the car was still a little down on power and I was getting a little more blow by out of the oil filler cap when the engine was pulling hard. I drove the car 35 miles home and the noise never came back. I checked the compression and found one cylinder with 30 lbs and the others are at 90. I took the intake off and saw nothing wrong with the lifters are valves. I took the head off and saw nothing wrong with no marks on the head, cylinder, or piston. Before I took the head off I poured some transmission oil into the closed intake valve hole to see if it would leak off. It did not leak off showing a good seal. When I turned the motor over it shot the fluid about three feet out from the car. I checked the compression again after the oil came out and it was 120. After I let the engine set for a couple hours I checked the compression again on that cylinder and it was back to the 30 lbs. I don't see anything wrong and don't know how to check for anything else. What do you think? Compression ring broken or something?