I was trying to get data on how about vacuum would created by a cylinder falling through the air, and ran across this:
http://astron.berkeley.edu/~jrg/ay202/node21.html
Excerpt:
"What happens for still larger Reynolds numbers? As we increase the speed further, the wake increases in size again and the drag increases. Experiments which go up to or so-indicate that a new periodicity appears in the wake, either because the whole wake is oscillating back and forth in a gross motion or because some new kind of vortex is occurring together with an irregular noisy motion. The details are as yet not entirely clear, and are still being studied experimentally. "

I wondered about the possible sound. And I thought of SDAI-Tech1.
[When "vortex" and "not entirely clear" collide, how can I not think of SDAI-Tech1?]
:)