The stepper motor drivers I'm using (L297/L298 combo, kit built from pminmo.com) emits a very loud whine, presumably from the current-limiting chopper circuit.

I tested another driver with better results ("Dragon Driver")

http://cgi.ebay.com/BIPOLAR-STEPPER-MOTOR-DRIVER-SINGLE-AXIS-Dragon-driver_W0QQitemZ260252305640QQihZ016QQcategoryZ71394QQrdZ1QQssPageNameZWD1VQQcmdZViewItem

With this drive the motor seems to be operating with no audible whine. However, a co-worker pointed out to me there could be a lot of near-ultrasonic energy being emitted just out of hearing range, which could still be dangerous.

I was thinking of looking for a sound level meter that can measure into the ultrasonic range. What is the consensus out there? Has anyone thought about this at length?

Might be worth the trouble. I've lost a little of my high frequency hearing (being the Air Force, and up til recently working on & around F-18s) and once its gone, its gone.