[mrob]

It's a closer analogy to the "beat frequencies" you get with constructive and destructive interference of waves of similar frequency. The envelope of the total amplitude of the pendulums oscillates with lower frequency than the amplitude of any individual pendulum.

I used Audacity to mix sine waves of the frequencies of the pendulums (51/60Hz, 52/60Hz, ..., 65/60Hz), multiplied by 440 to get them to audio frequency, and it sounds a lot like the UFO sound effect from the 1970s TV series "UFO".

[hrayr]

I'm very curious how that sounds, would you mind posting it somwhere?

[mrob]

Posting on first Google result for "share short sound clip". I can put it somewhere else if you have a better suggestion.

https://clyp.it/rfu2x1kb