Today I'm test sonar my Arducopter in ALT Hold Mode. I use USB HUD cable (Have Shield) connect to Pitot port IMU and receive signal from AN (pin 3) the Sonar. It perfect not noise in test.
Now, I use code ArduCopter2.0.39b
Wiring shield to ground at Sonar.
And connector to Pitot port IMU.
Installer to sonar mount under main frame.