When I was studying lasers, our lab had a pumped laser which could excite a variety of materials. We had a dye of some sort which was putting out a blue/green beam. It would put a hole in a thin sheet of metal. But it was more interesting that it produced a humming as it cut the metal, around 2 Khz, and it could be varied by changing the pumps strength or the material it was pumping. Even the proof didn't know what was making it.
I know how lasers work only in the most general terms. I've seen the amplitude of lasers modulated the same way as AM radio to transmit an audio signal. Was the laser you were studying being modulated in any way? Any modulation of either frequency or amplitude could cause the metal to vibrate, which would account for the sound you heard. Of course, it's possible a power/amplitude modulation could have been unintended...
Reggie Bautista
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