Chris, around 1980 I had a situation that sounds sort of similar. One thing: the noise showed up suddenly and it wasn't exactly constant. At low speed and at idle, I didn't notice it. It turned out to be a single valve spring that had broken oddly at the very final spring winding (not where I would expect that to ever happen and I've never seen one break like that since). It caused the spring to continue to "sort of work" but not as it should. At least that's what I finally decided happened. It made the noise like you described but not for long enough for me to figure that part out and such a small part had broken I thought that might be why I didn't notice it running much differently. The same spring soon also broke pretty close to the center of the winding coil and when I removed it, there were three pieces...one strange small piece down at the bottom and then the two larger broken pieces when the spring finally gave up the ghost. All springs had been replaced when a valve job was done a couple of years before that and I figured one of them was defective. More proof of that idea: Around 1990 another of those springs broke but not down at the bottom. At that point I went ahead and replaced all of them. May not be your problem but good luck! Might also be a sticking valve, I suppose.
