Found this topic, which I knew nothing of, while surfing. Here is one of many papers:
Here's a quote:
Furthermore, the tone is independent of the
degree of muscle contraction. Thus the typical rumbling sound is produced in the forearm if
the load is 0.25 kg or 6.6 kg although the intensity of the sound changes by a factor of about
You can even make your own:!-Part-1:-Electronic-Board/

It would seem to be the biofeedback mechanism I've been looking for to test for tension. Comments?
snobbyish, yet maybe helpful.