This noise started in the engine spontaneously a couple of weeks ago, barely perceptible. At first it seemed like it was just premature detonation so I tried the usual fixes: Pulled the choke out a bit. It seemed to help a little at first but as time went on I had to choke it almost to death - just barely able to run - to benefit at all. Put larger jets in the carb. Again, at first seemed to help but not any more. Cranked the vacuum brake on the distributor down as far as it would go. Again, only short lived benefit. Same with putting the highest octane gas available in. No help.
It has always had the characteristic of premature detonation - only exists at full vacuum under power over 40 mph, not when accelerating nor decelerating. Now it's heard at fast idle without any load after getting up to stable RPM as heard in the video.
I'm afraid that it is sounding more and more like the sound I had last year for a short time just before a piston came apart and I had to have the engine rebuilt (21,000 miles ago).
What do you think?
Here's the noise:
http://youtu.be/1BXBh417e_w