most likely localized and abrupt thermal expansion of the material.
i've noticed the same phenomenon:
http://www.photonlexicon.com/forums/...604#post249604

Originally Posted by
Insanity
Here's one for all you scientists/professors out there.
I came across a strange phenomenon completely by accident the other day, so I captured it as best I can with my iPhone...
I was working on one of my RGB's and when I stood in the projected beam I heard a buzzing sound which appeared to be coming from the light. I thought I was imagining things, but I could replicate it very easily. I have no idea how or why this happens, so hopefully one of you PL geniuses can explain this for me...
I don't know what material the jumper is made of, I ripped out the label some time ago as it was uncomfortable... But nonetheless here is a video of what is happening, you'll probably need the sound quite high as the iPhone isn't the best of devices to record this on...
Would love to know what causes this...
Cheers
Mark
suppose you're thinkin' about a plate o' shrimp. Suddenly someone'll say, like, plate, or shrimp, or plate o' shrimp out of the blue, no explanation. No point in lookin' for one, either. It's all part of a cosmic unconciousness.