The sound could definately be an issue - but the real kicker is going to be the power consumption. I see the circuit uses a pair of prehistoric 741 op-amps - that's ~6.5mA. The ultrasonic output tranducer is driven directly by the output of a pair of 4049's which can sink/source a fair amount of current - I'd guess that'll be about 25 mA. Add up all the resistors and pre-amp and I'd guess you're looking at about 35 to 50mA total power consumption - quite a drain on the batts!!
Hi Coal! I a played around with some ultrasonic transmitters and receivers in the past. I was into robotics at one time and used a Ultrasonic sensor to detect objects in front of the robot. There is one major downfall of the system that is going to give you fits and probably prevent you from using it in a cam. It does not work well in windy situations. Since the ultrasonic sensor detects motion and not motion and heat like a PIR, it does not take much to set it off. Falling leaves, birds and insects, blowing branches, etc... will almost always set it off. This will be a big problem. Plus, the detection distance is only about 10 feet max. However, you may be able to come up with some clever ways to get around these limitations. Keep us updated.